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The Ultimate Q51 Guide [Expert Level]

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New post 21 Feb 2018, 18:53
[GMAT math practice question]

The length and width of a certain rectangle are in the ratio 4 to 5. The area of the rectangle is 2,000 square units. If the length and width of the rectangle are both increased by 10 units, what is the increase in the area of the rectangle?

A. 800
B. 900
C. 1,000
D. 1,100
E. 1,250

=>
Suppose the length is L = 4k and the width is W = 5k for some k. Then
L*W = 4k*5k = 20k^2 = 2,000.
k^2 = 100 and k = 10.
Thus, L = 4k = 40 and W = 5k = 50.
The area of the rectangle after increasing both the length and the width is (L+10)(W+10) = (40 + 10)(50 + 10) = 50*60 = 3,000 square units.
Thus, the increase in the area is 3,000 – 2,000 = 1,000 square units.

Therefore, the answer is C.

Answer : C
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New post 23 Feb 2018, 02:17
[GMAT math practice question]

The lengths of two sides of a certain triangle are 3 and 9. What is the length of the 3rd side of the triangle?

1) The perimeter of the triangle is 20
2) The length of the 3rd side of the triangle is a multiple of 4

=>

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.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

Assume x is the length of the 3rd side of the triangle.
Since we have 1 variable (x) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Note:
Since (using the triangle inequality) x > 9 + 3 = 12 and x < 9 – 3 = 6, we have 6 < x < 12.

Condition 1)
x + 3 + 9 = 20
Thus x = 8.
Condition 1) is sufficient.

Condition 2)
The only integer between 6 and 12 that is a multiple of 4 is 8.
Thus, x = 8.
Condition 2) is sufficient, too.

In addition, since conditions 1) and 2) are similar, D is most likely to be the answer.

Therefore, D is the answer.

Answer: D
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New post 25 Feb 2018, 18:45
[GMAT math practice question]

If uv<0, is u^99v^100<0?

1) u<0
2) v>0

=>

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.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

In an inequality that has the right-hand side equal to 0, even exponents can be ignored.
Asking if u^99v1^00<0 is equivalent to asking if u < 0: simply divide both sides by u^98v^100 (> 0). We only need to consider the variables with odd exponents. Since the exponent of u is 99, which is an odd number, we must have u < 0. In addition, since uv < 0 by the original condition, u < 0 implies that v > 0. Thus, condition 1) is equivalent to condition 2), and D is most likely to be the answer by Tip 1).

Therefore, the answer is D.

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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New post 26 Feb 2018, 22:32
If x, y, and z are positive integers, is x+y divisible by 7?

1) x+z is divisible by 7
2) y+z is divisible by 7

=>

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.

Since we have 3 variables (x, y and z) and 0 equations, E is most likely to be the answer. So, we should consider conditions 1) & 2) together first.

Conditions 1) & 2):
If x = 7, y = 7, and z = 7, the answer is ‘yes’, since x + y = 14 is divisible by 7.
If x = 1, y = 1, and z = 6, the answer is ‘no’, since x + y = 2 is not divisible by 7.
Thus, conditions 1) and 2) together are not sufficient.

Therefore, E is the answer.

Answer: E

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 27 Feb 2018, 20:19
[GMAT math practice question]

Which of the following is closest to 11^5*9^5–7*10^7?

A. 10^2
B. 10^7
C. 10^8
D. 10^9
E. 10^10


=>

Since 11 is close to 10 and 9 is close to 10, 11^5*9^5 is close to 10^5*10^5 = 10^10.
Since 7*10^7 is a relatively small number compared with 10^10, 11^5*9^5 – 7*10^7 is approximately equal to 10^10.

Therefore, the answer is E.

Answer: E
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New post 01 Mar 2018, 03:52
[GMAT math practice question]

For positive integer m and n, is 2m/3n an integer?

1) m is a multiple of 3
2) n is a multiple of 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.

Since we have 2 variables (m and n) and 0 equations, C 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):
m = 3, n = 2: 2m/3n = 6/6 = 1 is an integer.
m = 3, n= 4: 2m/3n = 6/12 = 1/2 is not an integer.

Since we don’t have a unique solution, both conditions together 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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New post 01 Mar 2018, 20:00
[GMAT math practice question]

What is the number of factors of a positive integer n?

1) n is a multiple of 7
2) n 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.

Since we have 1 variable (n) and 0 equations, D is most likely to be the answer. So, we should consider each of the conditions on their own first.

Condition 1)
n = 7 : The number of factors of n is 2.
n = 14 : The number of factors of n is 4.
Since the answer is not unique, condition 1) is not sufficient.

Condition 2)
n = p^1, where p is a prime number.
The number of factors on n is 1 + 1 = 2.
Thus, condition 2) is sufficient.

Therefore, the answer is B.

Answer: B

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 04 Mar 2018, 19:29
[GMAT math practice question]

What is the value of x+y?

1) x+y is between 50 and 60
2) x and y are prime numbers

=>

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.

Since we have 2 variables (x and y) and 0 equations, C 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):
x = 23, y = 29 : x + y = 52
x = 23, y = 31 : x + y = 54

Since we don’t have a unique solution, both conditions together are not sufficient.

Therefore, the answer is E.

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 06 Mar 2018, 04:01
[GMAT math practice question]

For the integers x and y, if xy is a multiple of 25, is y a multiple of 5?

1) x-y is a multiple of 5
2) x is a multiple of 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.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

Since xy = 25 =5^2, x is a multiple of 5 or y is a multiple of 5. If x is a multiple of 5, y = x – 5 is also a multiple of 5. Thus the condition 1) is sufficient.

Condition 2)
Since we don’t have any information from the condition 2), 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 06 Mar 2018, 19:21
[GMAT math practice question]

Which of the following is 6/7 times as far from 7/3 as is 17/6 from 2/3?

A. 88/21
B. 21/88
C. 77/20
D. 20/77
E. 78/21

=>

We have |x – 7/3| = (6/7)|17/6 – 2/3|.
|x - 7/3| = (6/7)(13/6)
x – 7/3 = ±(6/7)(13/6) = ±13/7
x = ±13/7 + 7/3
x = (39 + 49)/21 = 88/21 or x = (-39 + 49)/21=10/21

Therefore, the answer is A.

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

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New post 07 Mar 2018, 19:53
[GMAT math practice question]

Is x^y <0?

1) x is positive
2) y 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.

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

Question:
x^y <0
⇔ x < 0 and y is an odd integer.

Condition 1)
Since x > 0, x^y > 0, regardless of the value of y.
Since ‘no’ is also a unique answer by CMT (Common Mistake Type) 1, condition 1) is sufficient.

Condition 2)
If x = -1 and y = 3, x^y = (-1)^3 = -1 < 0, and the answer is ‘yes’.
If x = -1 and y = 2, x^y = (-1)^2 = 1 > 0, and the answer is ‘no’.
Condition 2) is not sufficient since the question does not have a unique answer.

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 08 Mar 2018, 18:07
[GMAT math practice question]

If n!/(n-2)!<100, what is the greatest possible value of n?

A. 8
B. 9
C. 10
D. 11
E. 12

=>

We must have
n! / (n-2)! = n(n-1) < 100.
If n = 10, n(n-1) = 10*9 = 90 < 100.
If n = 11, n(n-1) = 11*10 = 110 > 100.

10 is the greatest value of n for which n!/(n-2)! < 100.

Therefore, C is the answer.

Answer: C
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New post 11 Mar 2018, 18:35
[GMAT math practice question]

If x and y are integers, is x a multiple of 5?

1) x+y is a multiple of 5
2) x+2y is a multiple of 5

=>

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.

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

Conditions 1) & 2):
x = 2(x+y) – (x+2y) is a multiple of 5 since 2(x+y) and (x+2y) are multiples of 5.
Thus, both conditions 1) & 2) together 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)
If x = 5 and y = 5, x is a multiple of 5, and the answer is ‘yes’.
If x = 2 and y = 3, x is not a multiple of 5, and the answer is ‘no’.
Thus condition 1) is not sufficient.

Condition 2)
If x = 5 and y = 5, x is a multiple of 5, and the answer is ‘yes’.
If x = 1 and y = 2, x is not a multiple of 5, and the answer is ‘no’.
Thus condition 2) is not sufficient.

Therefore, the answer is C.

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 12 Mar 2018, 18:10
[GMAT math practice question]

If x=y-z, is x>y?

1) y<0
2) z<0

=>

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.

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

We can modify the question as follows:
x>y
⇔ x-y > 0
⇔ -z > 0 by the original condition
⇔ z < 0
This is the same as condition 2).

Condition 1)
If x = -1, y = -2, and z = -1, x > y, and the answer is ‘yes’.
If x = -2, y = -1, and z = 1, x < y, and the answer is ‘no’.
Condition 1) is not sufficient since the question does not have a unique answer.

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 13 Mar 2018, 18:11
[GMAT math practice question]

If neither x nor y is divisible by 3, which of the following could be the value of x^2+y^2?

A. 333
B. 334
C. 335
D. 336
E. 337

=>
Consider the squares of the integers that are not divisible by 3:
1^2 = 1, 2^2 = 4, 4^2 = 16, 5^2 = 25, 7^2 = 49, 8^2=64, ….
They all have a remainder of 1 when they are divided by 3.
Thus, the sum of the squares of two integers which are not divisible by 3 must have a remainder of 2 when it is divided by 3.

The only answer choice having this property is 335.

Therefore, C is the answer.

Answer: C
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New post 15 Mar 2018, 18:08
[GMAT math practice question]

If x + y > 0, is xy^2 + x^2y > 0?

1) x > y
2) xy > 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.

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

Question:
xy^2 + x^2y > 0
=> xy(x+y) > 0
=> xy > 0, since x + y > 0
So, the question asks if xy > 0.

Since xy > 1 > 0 can be derived from condition 2), it is sufficient.
Condition 1) gives us no information about the sign of xy.

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 18 Mar 2018, 23:23
[GMAT math practice question]

Is (x-1)^3< (x-1)?

1) x>-1
2) x<0

=>

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.

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

The question can be modified as follows:
(x-1)^3< (x-1)
=> (x-1)^3- (x-1) < 0
=> (x-1)((x-1)^2- 1) < 0
=> (x-1)((x-1)+1)((x-1)-1) < 0
=> x(x-1)(x-2) < 0
=> x < 0 or 1 < x < 2
So, the question asks whether x < 0 or 1 < x < 2.

Condition 1)
Since the set of the question doesn’t include that of condition 1), it is not sufficient.

Condition 2)
Since the set of the question includes that of condition 2), it is sufficient.

Therefore, the answer is B.

Answer: B
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The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 19 Mar 2018, 18:06
[GMAT math practice question]

What is the median value of the data displayed in the following frequency table?

Attachment:
a.png
a.png [ 2.35 KiB | Viewed 357 times ]


1) x=2
2) y=5

=>

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.

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

If y = 1, the median is x.
If y = 2, the median is ( x + 3 ) / 2
If 3 ≤ y ≤ 9, the median is 3.
If y = 10, the median is ( 3 + 4 ) / 2 = 3.5
If y ≥ 10, the median is 4.

Thus, the condition 2) is sufficient.

Therefore, the answer is B.

Answer: B
_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $99 for 3 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"

Math Revolution GMAT Instructor
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V
Joined: 16 Aug 2015
Posts: 6381
GMAT 1: 760 Q51 V42
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 20 Mar 2018, 18:41
[GMAT math practice question]

If r, s are positive integers, is r/s a terminating decimal?

1) 1/r^2 is a terminating decimal
2) 1/s^2 is a terminating decimal

=>

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.

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

We can modify the question as follows:
r/s is a terminating decimal if and only if s has no other prime factors than 2 and 5.
This is the same as condition 2). Condition 1) tells us nothing about the prime factors of s.

Therefore, B is the answer.


Answer: B

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
"Only $99 for 3 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
"Unlimited Access to over 120 free video lessons - try it yourself"

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Posts: 6381
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Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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New post 22 Mar 2018, 18:01
[GMAT math practice question]

If (n+2)!= n!(an^2+bn+c), then abc=?

A. 2
B. 3
C. 4
D. 6
E. 8

=>

(n+2)! = (n+2)(n+1)n! = (n^2 + 3n + 2)n!
So,
a = 1, b = 3, and c = 2.
Thus, abc = 6.

Therefore, D is the answer.

Answer: D
_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
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Re: The Ultimate Q51 Guide [Expert Level] &nbs [#permalink] 22 Mar 2018, 18:01

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