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Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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2x+3y=?

1) x=6
2) 4x+6y=18

==> In the original condition, you get 2x+3y=?. For con 2), if you divide both sides by 2, you get 2x+3y=9, hence it is unique and sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If 3 juniors from 8 juniors are selected at random to make a committee, how many cases are possible?

A. 8
B. 28
C. 35
D. 42
E. 56

==> In general, you solve probability questions using nCr, a combination. In other words, from 8C3=56, the answer is E.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If $$x^{-2}+x^{-1}=0$$, x=?

A. -1
B. 0
C. 1
D. 0, 1
E. -1, 0

==> If you multiply $$x^2$$ on both sides, you get 1+x=0, x=-1. The answer is A.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If b≠0, |a/b|=a/b, is b>0?

1) a>0
2) a+b>0

==> If you modify the original condition and the question, in order to satisfy |a/b|=a/b, you must get a/b≥0, and if you multiply b^2 on both sides, from (b^2)a/b≥(b^2)0, you get ab≥0. Since the question is b>0? becomes a≥0?, con 1) is yes and sufficient. For con 2), using CMT 4 (B: if you get A or B too easily, consider D), you only get ab≥0 and a+b>0 when a>0 and b>0, hence yes, it is sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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When n and k are positive integers, what is the greatest common divisor of n+k and n?

1) n=2
2) k=1

==> In the original condition, there are 2 variables, and in order to match the number of variables to the number of equations, there must be 2 equations. Since there is 1 for con 1) and 1 for con 2), C is most likely to be the answer. By solving con 1) and con 2), you get n+k=2+1=3 and n=2, and GCD(3,2)=1, hence it is unique and sufficient. Therefore, the answer is C. However, this is an integer question, one of the key questions, so you apply CMT 4 (A: if you get C too easily, consider A or B).
For con 1), k is unknown hence it is not sufficient.
For con 2), if k=1, n+k(=n+1) and n becomes 2 consecutive integers, so always GCD=1, hence it is unique and sufficient.

Therefore, the answer is B, not C.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If x and y are positive, is (y/x)+(x/y)>2?

1) x>y
2) x>1>y

==> If you modify the original condition and the question, even if you multiply xy on both sides, you get xy>0, hence the inequality sign doesn’t change. Thus, you get is (y/x)+(x/y)>2?, or $$y^2+x^2>2xy?$$, or $$y^2+x^2-2xy>0$$?, or $$(x-y)^2>0$$?, or x≠y?. From con 1) = con 2), it is always yes and sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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When a certain coin is flipped, the probability that the coin will land on head or tail is 1/2 each. If the coin is flipped 4 times, what is the probability that it will land on tail at least twice on 4 flips?

A. 3/8
B. 1/16
C. 1/2
D. 5/8
E. 11/16

==> In general, you solve probability questions using nCr combination. In other words, from TTHH, there are (4!/2!/2!)=6 possibilities, so you get (1/2)(1/2)(1/2)(1/2)*6=3/8. From TTTH, there are (4!/3!)=4 possibilities, so you get (1/2)(1/2)(1/2)(1/2)*4=1/4. From TTTT, there is only 1 possibility, so you get (1/2)(1/2)(1/2)(1/2)=1/16 and (3/8)+(1/4)+(1/16)=(6+4+1)/16=11/16. The answer is E.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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There are a lot of products. Is the standard deviation of the prices of the products less than $60? 1) The median value of their price is$100
2) The range of their price is $110 ==> If you modify the original condition and the question, you get standard deviation(d)≤range/2. Then, from d≤range/2=$110/2=$55<$60, it is always yes and sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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When a/b=1.2, (a-b)/(a+b)=?

A. 1/15
B. 1/11
C. 1/9
D. 3/8
E. 1/3

==> From a/b=1.2=6/5, if you substitute a=6 and b=5, you get (a-b)/(a+b)=(6-5)/(6+5)=1/11.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If nk≠0, n is what percent of k?

1) k=0.2n
2) (k+n)/n=1.2

==> If you modify the original condition and the question, from n=(some)(1/100)k, you get some=100(n/k). However, con 1) = con 2), so you get some=100(0.2)=20, hence it is unique and sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If the sum of the annual salary of n persons is $x and the monthly salary per person is$y, what is the value of n in terms of x and y?

A. $x/12y B.$12x/y
C. $12xy D.$12y/x
E. \$xy/12

==> If the annual salary per person: s, you get ns=x, s=12y. If you substitue this, you get n(12y)=x, n=x/12y.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If n is the product of 3 consecutive integers, which of the following must be true?

I. a multiple of 2 II. a multiple of 3 III. a multiple of 4

A. I only
B. II only
C. III only
D. I and II only
E. II and III only

==> From n=m(m+1)(m+2) , where m=integer, the product of 3 consecutive integers is always a multiple of 6. Thus, I and II is always true. For III, if m=1, from n=(1)(2)(3)=6, it is not a multiple of 4, hence it is false.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If the average (arithmetic mean) of set A is 10,000 and the average (arithmetic mean) of set B is 10,000, what is the range of set A and set B combined?

1) The range of set A is 6,000
2) The range of set B is 3,000

==> If you modify the original condition and the question, since there are 2 sets, set 1’s range=set 1’s Max-set 1’s min, and set 2’s range=set 2’s Max-set 2’s min. Thus, there are 6 variables and 2 equations, and in order to match the number of variables to the number of equations, there must be 4 more equations. Since there is 1 for con 1) and 1 for con 2), E is most likely to be the answer. By solving con 1) and con 2), the max and the min when combined is unknown, hence it is not sufficient. Therefore, the answer is E.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If x+y=3 and $$x^2+y^2=7$$, then xy=?

A. -2
B. -1
C. 0
D. 1
E. 2

==> You get $$(x+y)^2=x^2+y^2+2xy$$, and if you substitute this, from $$3^2=7+2xy$$, you get 2xy=2, and xy=1.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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(1/7)+(1/8)+(1/9) is in between?

A. (1/6) and (1/5)
B. (1/5) and (1/4)
C. (1/4) and (1/3)
D. (1/3) and (1/2)
E. (1/2) and 1

==>The sum of consecutive reciprocal numbers is decided by the first and the last number. In other words, you get
(1/9)+(1/9)+(1/9)< (1/7)+(1/8)+(1/9)< (1/7)+(1/7)+(1/7), and if you reorganize this, from
=1/3=3/9=(1/9)+(1/9)+(1/9)<(1/7)+(1/8)+(1/9)<(1/7)+(1/7)+(1/7)=3/7<3/6=1/2, you get between (1/3) and (1/2).

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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Set S has six numbers and their average (arithmetic mean) is 32. What is the median of the numbers?

1) The six numbers are greater than or equal to 31
2) There is “37” in set S

==> In the original condition, there are 6 variables and 1 equation. In order to match the number of variables to the number of equations, there must be 5 more equations. Since there is 1 for con 1) and 1 for con 2), E is most likely to be the answer. By solving con 1) and con 2), you get
31, 31, 31, 31, 31, 37, and so the median=31+310/2=31, hence it is unique and sufficient.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If a prime number m can be expressed as $$2^n-1$$, where n is a positive integer, which of the following can be the value of m?

A. 11
B. 15
C. 31
D. 63
E. 97

==> If n=5, you get $$2^5-1=31$$. The answer is C.
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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For a positive integer n, when 12n is divided by 15, which of the following cannot be the remainder?

A. 0
B. 3
C. 5
D. 6
E. 9

==>From 12n=15Q+r, 12 and 15 are the multiples of 3, so remainder=r must also be the multiple of 3. Therefore, 5 cannot be the remainder.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
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G(x) is the greatest integer less than or equal to x and L(x) is the least integer greater than or equal to x. When x is not an integer, which of the following is the value of L(x)-G(x)?

A. -2
B. -1
C. 0
D. 1
E. 2

==> You get G(x)=round down and L(X)=round up. Then, x≠integer, so if you substitute x=1.2, you get L(1.2)-G(1.2)=2-1=1.

_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8123
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: The Ultimate Q51 Guide [Expert Level]  [#permalink]

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If the average (arithmetic mean) of a, b, and c is (a+2b)/3, what is the value of b?

1) a=1
2) c=2

==> If you modify the original condition and the question and check the question again, from is (a+2b)/3=(a+b+c)/3, you get a+2b=a+b+c, then b=c. In order to find b, you only need to know c.

_________________ Re: The Ultimate Q51 Guide [Expert Level]   [#permalink] 17 May 2017, 17:55

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# The Ultimate Q51 Guide [Expert Level]

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