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The Ultimate Q51 Guide [Expert Level]

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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 20 Oct 2019, 18:15
[GMAT math practice question]

(function) f(x) is a function. What is the value of f(2006)?

1) f(11)=11
2) f(x+3)=(f(x)-1)/(f(x)+1)

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have many variables to determine a function and 0 equations, E is most likely to be the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)

Since we have f(x+3) = (f(x)-1)/(f(x)+1) and f(11)=11, we have f(14)=(f(11)-1)/(f(11)+1)=10/12=5/6 when we substitute 11 for x.
We have f(17) = (f(14)-1)/(f(14)+1) = ((5/6)-1)/(5/6)+1) = (-(1/6))/(11/6) = -1/11 when we substitute 14 for x.
We have f(20) = (f(17)-1)/(f(17)+1) = (-(1/11)-1)/(-(1/11)+1) = (-(12/11))/(10/11) = -12/10 = -6/5, when we substitute 17 for x.
We have f(23) = (f(20)-1)/(f(20)+1) = (-(6/5)-1)/(-(6/5)+1) = (-(11/5))/(-(1/5))=11, when we substitute 20 for x.
Then we have the following patterns.
f(11) = f(23) = f(35) = … = f(12k-1) = 11
f(14) = f(26) = f(38) = … = f(12k+2) = 5/6
f(17) = f(29) = f(41) = … = f(12k+5) = -1/11
f(20) = f(32) = f(44) = … = f(12k+8) = -6/5
So, f(2006) = f(12*167+2) = 5/6.

Therefore, C is the answer.
Answer: C

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions when the answer is A, B, C, or D.
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New post 21 Oct 2019, 19:02
[GMAT math practice question]

(Number) What is a positive integer p?

1) p is a prime number
2) p^2+2 is a prime number

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 1 variable (p) and 0 equations, D is most likely to be the answer. So, we should consider each condition on its own first.

Condition 1)
Since we have an infinite number of prime numbers, we don’t have a unique value of p, and condition 1) is not sufficient.

Condition 2)
If p has a remainder 1 when it is divided by 3 or p=3k+1 for some integer k, then p^2+2 = (3k+1)^2+2 = 9k^2+6k+1+2 = 3(3k^2+2k+1) is a multiple and it is a prime number. We have 3k^2+2k+1=1, 3k^2+2k=0, k(3k+2)=0 and k=0 or k=-2/3. However, k is an integer so only k=0 works. Then p=3(0)+1 = 1. However, p = 1 is not a solution since 1 is not a prime number.

If p has a remainder 2 when it is divided by 3 or p=3k+2 for some integer k, then p^2+2 = (3k+2)^2+2 = 9k^2+12k+4+2 = 3(3k^2+4k+2) is a multiple and it is a prime number. Since we have 3k^2+4k+2=1, 3k^2+4k+1=0 or (3k+1)(k+1)=0 and we have k =-1 and k=-1/3. However, k must be an integer so then p=3(-1)+2 = -1. However, p = -1 is not a solution since -1 is negative.

Assume p has a remainder 0 when it is divided by 3.
If p=3, then p^2+2=11 is a prime number.
If p=9, then p^2+2=83 is a prime number.
Since condition 2) does not yield a unique solution, it is not sufficient.

Conditions 1) & 2)
p is a multiple of 3 from condition 2), and p is a prime number from condition 1). Then p = 3.
Since both conditions together yield a unique solution, it is sufficient.

Therefore, C is the answer.
Answer: C

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations,” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.
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New post 22 Oct 2019, 17:39
[GMAT math practice question]

(number properties) m and n are integers. What is the value (-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}?

1) m = n + 1
2) m = 3

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 2 variables (m and n) and 0 equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Since we have m = n + 1 and m = 3, we can substitute m = 3 into m = n + 1 to get 3 = n + 1 and n = 2.
(-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}
=(-1)^{3-2} +(-1)^{3+2} +(-1)^{3*2} +(-1)^{2*2}
=(-1)^1 +(-1)^5 +(-1)^6 +(-1)^4
=(-1) + (-1) + 1 + 1 = 0
Since both conditions together yield a unique solution, they are sufficient.

Since this question is an integer question (one of the key question areas), CMT (Common Mistake Type) 4(A) of the VA (Variable Approach) method tells us that we should also check answers A and B.

Condition 1)
Since m = n + 1, m and n are consecutive integers, they have different parities, which means that if m is an odd integer, then n is an even integer, and if m is an even integer, then n is an odd integer.

Case 1: m is an odd integer and n is an even integer.
Then, m+n is an odd integer, m – n is an odd integer, mn is an even integer and 2n is an even integer.
(-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}
=(-1)^{odd} +(-1)^{odd} +(-1)^{even} +(-1)^{even}
=(-1) + (-1) + 1 + 1 = 0

Case 2: m is an even integer and n is an odd integer.
Then, m+n is an odd integer, m – n is an odd integer, mn is an even integer and 2n is an even integer.
(-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}
=(-1)^{odd} +(-1)^{odd} +(-1)^{even} +(-1)^{even}
=(-1) + (-1) + 1 + 1 = 0

Since condition 1) yields a unique solution, it is sufficient.

Condition 2)

Case 1: n is an even integer.
Then, m+n is an odd integer, m – n is an odd integer, mn is an even integer and 2n is an even integer, since m = 3.
(-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}
=(-1)^{odd} +(-1)^{odd} +(-1)^{even} +(-1)^{even}
=(-1) + (-1) + 1 + 1 = 0

Case 2: n is an odd integer.
Then, m+n is an even integer, m – n is an even integer, mn is an odd integer and 2n is an even integer.
(-1)^{m-n} +(-1)^{m+n} +(-1)^{mn} +(-1)^{2n}
=(-1)^{even} +(-1)^{even} +(-1)^{odd} +(-1)^{even}
=1 + 1 + (-1) + 1 = 2

Since condition 2) does not yield a unique solution, it is not sufficient.

If the question has both C and A as its answer, then A is an answer rather than C by the definition of DS questions. Also, this question is a 50/51 level question and can be solved by using the Variable Approach and the relationship between Common Mistake Type 3 and 4 (A or B).

Therefore, A is the answer.
Answer: A

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
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New post 24 Oct 2019, 18:08
[GMAT math practice question]

(Geometry) The figure shows that line m is parallel to the line n, and l is parallel to k. Moreover, ∠CBD=45°, ∠FAE=80°. What is ∠BDC?

Attachment:
10.14ps.png
10.14ps.png [ 19.74 KiB | Viewed 349 times ]


A. 40° B. 45° C. 50° D. 55° E. 60°

=>

Attachment:
10.21PS(A).png
10.21PS(A).png [ 29.3 KiB | Viewed 349 times ]


Since lines m and n are parallel, we have <FBG=<FAE=80° and 80°+<ABD+45°=180°.
Then we have 125°+<ABD=180 and <ABD=55°.
Since <ABD and <BDC are alternate interior angles, they are congruent and <BDC=55°.

Therefore, D is the answer.
Answer: D
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New post 27 Oct 2019, 18:54
[GMAT math practice question]

(number properties) p and q are integers. Is (p-1)(q-1) an even number?

1) p+q is an odd number
2) pq is an even number

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

The following reasoning shows that in the question, either p or q is an odd integer.
(p-1)(q-1) is an even integer
=> p – 1 or q – 1 is an even integer
=> p or q is an odd integer

Therefore, either p and q is an odd number, and the other one is an even number, according to condition 1. So, condition 1) is sufficient.

Condition 2)

If p is an odd number and q is an even number, then p-1 is an even number, q-1 is an odd number, and (p-1)(q-1) is an even number, which means the answer is ‘yes’.
If both p and q are even numbers, then (p-1)(q-1) is an odd number, and the answer is ‘no’ since both p-1 and q-1 are odd numbers.

Since condition 2) does not yield a unique solution, it is not sufficient.

Therefore, A is the answer.
Answer: A
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New post 28 Oct 2019, 22:34
[GMAT math practice question]

(geometry) The figure shows that OA = 20, OB = 30 and OC = x and □OCDE is a rectangle. What is the area of rectangle OCDE?

1) x = 10
2) OE = 15

Attachment:
10.23DS.png
10.23DS.png [ 10.14 KiB | Viewed 309 times ]


=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

Since the triangle OAB and the triangle CAD are similar, we have OA:OB = 2:3 and CA:CD = 2:3. Then we have 3CA = 2CD or CD = (3/2)(20-x).
So the area of the rectangle OCDE is x*(3/2)(20-x). Therefore, we have one variable in this question.

Since we have 1 variable (x) and 0 equations, D is the most likely answer. So, we should consider each condition separately first.

Condition 1) is sufficient, since it yields a unique solution.


Condition 2)
Since CD = OE = 15 from condition 2), and from the original condition we know CD = (3/2)(20-x).
=>15 = (3/2)(20-x)
=>10 = 20-x
=>x = 10
=>3CA = 2CD
=>3CA = 2(15)
=>3CA = 30
=>CA = 10
We have CA = 10 and x = 10.
So, condition 2) is also sufficient, because it is equivalent to condition 1).

Therefore, D is the answer.
Answer: D

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.
_________________
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 29 Oct 2019, 17:33
[GMAT math practice question]

(propotional) What is z^2/xy + x^2/yz + y^2/zx ?

1) x:y = 2:3
2) x:z = 1:2

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 3 variables (x, y, and z) and 0 equations, E is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Since x:y = 2:3 and x:z = 1:2, we have x:y:z = 2:3:4.
Then we have x = 2k, y = 3k, and z = 4k for some number k.
z^2/xy + x^2/yz + y^2/zx
= (4k)^2/(2k)(3k) + (2k)^2/(3k)(4k) + (3k)^2/(4k)(2k)
= 16k^2/6k^2 + 4k^2/12k^2 + 9k^2/8k^2
= 16/6 + 4/12 + 9/8
= 64/24 + 8/24 + 27/24 = 99/24 = 33/8.

Since both conditions together yield a unique solution, they are sufficient.

Therefore, C is the answer.
Answer: C

In cases where 3 or more additional equations are required, such as for original conditions with “3 variables”, or “4 variables and 1 equation”, or “5 variables and 2 equations”, conditions 1) and 2) usually supply only one additional equation. Therefore, there is an 80% chance that E is the answer, a 15% chance that C is the answer, and a 5% chance that the answer is A, B or D. Since E (i.e. conditions 1) & 2) are NOT sufficient, when taken together) is most likely to be the answer, it is generally most efficient to begin by checking the sufficiency of conditions 1) and 2), when taken together. Obviously, there may be occasions on which the answer is A, B, C or D.
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New post 30 Oct 2019, 17:24
[GMAT math practice question]

(equation) What are the values of x+y and xy?

1) x + y + xy = -2
2) (1/x) + (1/y) = 1

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have 2 variables (x and y) and 0 equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Since 1/x + 1/y = 1 from condition 2), we have y + x = xy by multiplying both sides of the equation by xy, which rearranges to get xy – (x+y) = 0.
Since xy + (x+y) = -2 from condition 1), we have xy - (x+y) + xy + (x+y) = 0 + -2 by adding the two equations. Then 2xy = -2 or xy = -1.
Then we have x+y=-1.

Since both conditions together yield a unique solution, they are sufficient.

Therefore, C is the answer.
Answer: C

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
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New post 31 Oct 2019, 17:38
[GMAT math practice question]

(geometry) The figure shows the rectangle ABCD. What is ∠x - ∠y?

Attachment:
10.23PS.png
10.23PS.png [ 20.7 KiB | Viewed 236 times ]


A. 20°
B. 17°
C. 15°
D. 13°
E. 12°

=>

Attachment:
10.23ps(a).png
10.23ps(a).png [ 21.61 KiB | Viewed 236 times ]


Since AP, RP and CD are parallel, we have <ABP = <BPR and <DQP = <RPQ. Since <BPQ = <BPR + <RPQ, we have <x = 15° + <y.
So, we have <x - <y = 15°.

Therefore, C is the answer.
Answer: C
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New post 03 Nov 2019, 18:39
[GMAT math practice question]

(algebra) What is the value of x/(x+y) + y/(x-y)?

1) (x+y):y = 3:1
2) x + y = 8

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

The question x/(x+y) + y/(x-y) is equivalent to (x^2+y^2)/(x^2-y^2) for the following reason
x/(x+y) + y/(x-y)
=> x(x-y)/(x+y)(x-y) + y(x+y)/(x+y)(x-y)
=> (x^2-xy+xy+y^2)/(x^2-y^2)
=> (x^2+y^2)/(x^2-y^2)
=> (x^2/y^2+1)/(x^2/y^2-1) by dividing the top and bottom by y^2
=> [(x/y)^2+1)/[(x/y)^2-1]

When a question asks for a ratio, if one condition includes a ratio and the other condition just gives a number, the condition including the ratio is most likely to be sufficient. This tells us that A is most likely to be the answer to this question.

Condition 1)
The condition (x+y):y = 3:1 is equivalent to x = 2y since x + y = 3y from (x+y):y = 3:1.
Then (x^2+y^2)/(x^2-y^2) = ((2y)^2+y^2)/((2y)^2-y^2) = (4y^2+y^2)/(4y^2-y^2) = 5y^2/3y^2 = 5/3.
Since condition 1) yields a unique solution, it is sufficient.


Condition 2)
If x = 5 and y = 3, then we have x/(x+y) + y/(x-y) = 5/8 + 3/2 = 5/8 + 12/8 = 17/8.
If x = 6 and y = 2, then we have x/(x+y) + y/(x-y) = 6/8 + 2/4 = 3/4 + 2/4 = 5/4.
Since condition 2) does not yield a unique solution, it is not sufficient.

Therefore, A is the answer.
Answer: A
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 04 Nov 2019, 17:42
[GMAT math practice question]

(algebra) max{x, y} denotes the maximum of x and y, and min{x, y} denotes the minimum of x and y. What is the value of x + y?

1) max{x, y} = x + y
2) min{x, y} = 2x + y - 2

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
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Since we have 2 variables (x and y) and 0 equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Case 1: x ≥ y
Since max(x,y) = x and max(x,y) = x + y, we have x = x + y or y = 0.
Since min(x,y) = y and min(x,y) = 2x + y - 2, we have y = 2x + y - 2, 0 = 2x - 2, 2x = 2, or x = 1.
Then we have x + y = 0 + 1 = 1.
Case 2: x < y
Since max(x,y) = y and max(x,y) = x + y, we have y = x + y or x = 0.
Since min(x,y) = x, min(x,y) = 2x + y - 2 and x = 0, we have x = 2x + y - 2 or y = 2.
Then we have x + y = 0 + 2 = 2.

Since both conditions together do not yield a unique solution, they are not sufficient.

Therefore, E is the answer.
Answer: E

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 05 Nov 2019, 18:11
[GMAT math practice question]

(equation) What is the value of a + b?

1) ax + by = 2(ax - by) - 3 = x + y + 7
2) x = 3, y = 1

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

We have 4 variables (a, b, x and y). However, since both conditions have 4 equations, C is most likely the answer. So, we should consider conditions 1) & 2) together first. After comparing the number of variables and the number of equations, we can save time by considering conditions 1) & 2) together first.

Conditions 1) & 2)
Since x = 3 and y = 1, we have 3a + b = 2(3a - b) - 3 = 3+1+7 = 11.
Then we have 3a + b = 11 and 6a - 2b = 14 or 3a – b = 7.
When we add those equations we have 3a + b + 3a - b = 11 + 7, 6a = 18 or a = 3.
Then we have 3(3) + b = 11, 9 + b = 11 or b = 2.

Since both conditions together yield a unique solution, they are sufficient.

Therefore, C is the answer.
Answer: C

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B, or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D, or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 06 Nov 2019, 20:35
[GMAT math practice question]

(geometry) The figure shows that triangle ABC is inscribed in circle O. If ∠AOB = x + 60°, ∠BOC = 2x + 20°, ∠AOC = 3x - 20°, what is ∠ACB?

Attachment:
10.28ps.png
10.28ps.png [ 16.76 KiB | Viewed 535 times ]


A. 45°
B. 50°
C. 55°
D. 60°
E. 65°


=>

Since the sum of central angles of those three sectors equals to 360°, we have x + 60°+ 2x + 20°+ 3x - 20°= 360°, 6x + 60°= 360°, 6x = 300°, or x = 50°
Then we have ∠AOB = x + 60° = 50° + 60° = 110°, ∠BOC = 2x + 20° = 2(50°) + 20° = 120°, ∠AOC = 3x - 20° = 3(50°) - 20° = 130°.

Attachment:
10.28ps(a).png
10.28ps(a).png [ 16.45 KiB | Viewed 528 times ]


∠ACO = (180°- 130°)/2 = 25° and ∠BCO = (180°- 120°)/2 = 30°.
Then ∠ACB = ∠AOC + ∠BOC = 25°+ 30°= 55°.

Therefore, C is the answer.
Answer: C
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New post 07 Nov 2019, 18:36
[GMAT math practice question]

(probability) The figure shows that l is parallel to m and points A, B, C, D, E are on line l, and points F, G, H, I are on the line m. How many quadrilaterals are possible with 4 points out of the above 8 points?

A.48
B. 56
C. 60
D. 72
E. 108

Attachment:
10.30ps.png
10.30ps.png [ 15.36 KiB | Viewed 508 times ]


=>

To create a quadrilateral, we should choose 2 points out of 5 points on line l, and 2 points out of 4 points on the line m and connect them.
Then we have 5C2*4C2 = [(5*4)/(1*2)][(4*3)/(1*2)] = 10*6 = 60.

Therefore, C is the answer.
Answer: C
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New post 10 Nov 2019, 20:32
[GMAT math practice question]

b = (-1)+(-1)^2+(-1)^3+….+(-1)^a. What is the value of b?

1) a = 2019
2) a is an odd number.

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

Since we have \(2\) variables (\(a\) and \(b\)) and \(1\) equation, D is most likely to be the answer. So, we should consider each condition on its own first.

Condition 1)
We have \((-1) = (-1)^3 = (-1)^5 = … =(-1)^{2019} = -1 and (-1)^2 = (-1)^4 = (-1)^6 = … = (-1)^{2018} = 1. (-1) + (-1)^2 + (-1)^3+….+(-1)^a. = ((-1)+(-1)^2) + ((-1)^3+(-1)^4) + … + ((-1)^{2017} +(-1)^{2018}) + (-1)^{2019} = 0 + 0 + … + 0 + (-1) = -1\)

Since condition 1) yields a unique solution, it is sufficient.

Condition 2)
We have \((-1) = (-1)^3 = (-1)^5 = … =(-1)^a = -1 and (-1)^2 = (-1)^4 = (-1)^6 = … = (-1)^{a-1} = 1. (-1) + (-1)^2 + (-1)^3+….+(-1)^a. = ((-1)+(-1)^2) + ((-1)^3+(-1)^4) + … + ((-1)^{a-2} +(-1)^{a-1}) + (-1)^a = 0 + 0 + … + 0 + (-1) = -1\)

Since condition 2) yields a unique solution, it is sufficient.

Therefore, D is the answer.
Answer: D

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A, B, C or E.
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 11 Nov 2019, 21:20
[GMAT math practice question]

(algebra) What is the value of (4x-3xy+4y)/(3x+3y)?

1) x = y
2) (1/x) + (1/y) = 3

=>


Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

The question asks the value of (4/3) – xy/(x+y) for the following reason
(4x-3xy+4y)/(3x+3y)
= [4(x+y)-3xy]/[3(x+y)]
= [4(x+y)]/[3(x+y)] - [3xy]/[3(x+y)]
= (4/3) – xy/(x+y)

Since we have (x+y)/xy = 3 from condition 2),) for the following reason
(1/x)+(1/y) = 3
(y/xy) + (x/xy) = 3
(x+y)/xy = 3
Then we have xy/(x+y) = 1/3.
Then (4/3) – xy/(x+y) = (4/3) – (1/3) = 1.

Since condition 2) yields a unique solution, it is sufficient.

Condition 1)
Since we have x=y, (4/3) – xy/(x+y) = (4/3) – x^2/2x = (4/3)-x/2.
If x = y = 1, then (4/3) - x/2 = 5/6.
If x = y = 2, then (4/3) - x/2 = 1/3.

Since condition 1) does not yield a unique solution, it is not sufficient.

Therefore, B is the answer.
Answer: B
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 12 Nov 2019, 18:37
[GMAT math practice question]

(Equation) 2ax - 3b = a - bx is an equation in terms of x. What is its solution?

1) -3/2 is a solution of (b-a)x - (2a-3b) = 0
2) a = 3b

=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.
The question asks the value of (a+3b)/(2a+b) for the following reason.
2ax - 3b = a - bx
=> 2ax + bx = a + 3b
=> x(2a+b) = a+3b
=> x = (a+3b)/(2a+b)

Since we have a = 3b from condition 2), we have x = (a+3b)/(2a+b) = (3b+3b)/(6b+b) = (6b)/(7b) = 6/7.
Thus, condition 2) is sufficient.

Condition 1)
When we substitute -3/2 for x, we have (b-a)(-3/2) - (2a-3b) = 0 or (-3)(b-a) = 2(2a-3b). We have -3b+3a = 4a-6b or a = 3b.
Condition 1) is equivalent to condition 2), and it is also sufficient.

Therefore, D is the answer.

When a question asks for a ratio, if one condition includes a ratio and the other condition just gives a number, the condition including the ratio is most likely to be sufficient. This tells us that D is most likely to be the answer to this question, since each condition includes a ratio.

Note: Tip 1) of the VA method states that D is most likely to be the answer if condition 1) gives the same information as condition 2).

This question is a CMT4(B) question: condition 2) is easy to work with, and condition 1) is difficult to work with. For CMT4(B) questions, D is most likely to be the answer.
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 13 Nov 2019, 17:41
[GMAT math practice question]

The figure shows that l is parallel to m, ABCD is a square, and the point D is on l and B is on m. What is ∠y - ∠x?

A. 15°
B. 25°
C. 30°
D. 35°
E. 40°

Attachment:
11.4.png
11.4.png [ 20.8 KiB | Viewed 455 times ]


=>

When we draw an additional line k on the figure as follows,

Attachment:
11.4ps(a).png
11.4ps(a).png [ 37.31 KiB | Viewed 455 times ]


Since ∠BCD = 90°, we have ∠x + 8∠x = 90° and ∠x = 10°.
Since ∠x + ∠y = 45°, we have ∠y = 35°. Thus ∠y - ∠x = 25°.

Therefore, B is the answer.
Answer: B
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 14 Nov 2019, 17:41
[GMAT math practice question]

(geometry) The figure shows that the lines AB and AC are the tangential lines to the circle O. What is ∠ABC?

Attachment:
11.5ps.png
11.5ps.png [ 11.72 KiB | Viewed 443 times ]


A. 55
B. 60
C. 65
D. 70
E. 75

=>

Since AB and AC are tangent lines to the circle, we have AB = AC.
Since the triangle ABC is isosceles with AB = AC, we have ∠ABC = (1/2)(180-∠BAC) = 70.

Therefore, D is the answer.
Answer: D
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 17 Nov 2019, 17:43
[GMAT math practice question]

(number properties) p and q are positive integers and relative primes. Is p divisible by 1979?

1) p is a multiple of 1979.
2) p/q = 1 - (1/2) + (1/3) - (1/4) +…- (1/1318) + (1/1319).

=>


Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.
Visit https://www.mathrevolution.com/gmat/lesson for details.

The first step of the VA (Variable Approach) method is to modify the original condition and the question. We then recheck the question. We should simplify conditions if necessary.

The question “is p divisible by 1979” is equivalent to condition 1) “p is a multiple of 1979”.

Condition 2)
Remember that -1/2k = 1/2k – 1/k for k = 1, 2, 3, …, 659.
-1/2 = 1/2 – 1/1
-1/4 = 1/4 – 1/2
-1/6 = 1/6 – 1/3

-1/1318 = 1/1318 – 1/659
p/q = 1 + 1/2 + 1/3 + 1/4 + … + 1/1317 + 1/1318 + 1/1319 – 2(1/2 + 1/4 + … + 1/1318)
= 1 + 1/2 + 1/3 + 1/4 + … + 1/1317 + 1/1318 + 1/1319 – (1/1 + 1/2 + … + 1/659)
= 1/660 + 1/661 + … + 1/1318 + 1/1319
= (1/660 + 1/1319) + (1/661 + 1/1318) + … + (1/989 + 1/990)
= 1979/(660*1319) + 1979/(661*1318) + … + 1979/(989*990)
= (1979*k)/(660*661*…*1318*1319)

Then, we have p(660*661*…*1318*1319) = q(1979*k).
Since 1979 is a prime number, p is a multiple of 1979.

Therefore, D is the answer.
Answer: D

This question is a CMT4 (B) question: condition 1) is easy to work with, and condition 2) is difficult to work with. For CMT4 (B) questions, D is most likely to be the answer.
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Re: The Ultimate Q51 Guide [Expert Level]   [#permalink] 17 Nov 2019, 17:43

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