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Is a=0?

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Math Revolution GMAT Instructor
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Joined: 16 Aug 2015
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Is a=0? [#permalink]

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New post 28 Dec 2017, 00:41
00:00
A
B
C
D
E

Difficulty:

  45% (medium)

Question Stats:

68% (00:53) correct 32% (00:36) wrong based on 63 sessions

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[GMAT math practice question]

Is \(a=0?\)

1) \(ab=-3a\)
2) \(b>0\)

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Re: Is a=0? [#permalink]

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New post 28 Dec 2017, 06:00
MathRevolution wrote:
[GMAT math practice question]

Is \(a=0?\)

1) \(ab=-3a\)
2) \(b>0\)


1) \(ab=-3a....ab+3a=0.....a(b+3)=0\)
so either a is 0 or b=-3 or both
insuff

2) \(b>0\)
Nothing about a..
insuff

combined..
b cannot be -3, so a=0
suff
C
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1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html
3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html


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Expert Post
Math Revolution GMAT Instructor
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Joined: 16 Aug 2015
Posts: 5845
GMAT 1: 760 Q51 V42
GPA: 3.82
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Re: Is a=0? [#permalink]

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New post 01 Jan 2018, 02:10
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) and 2) together first.

Conditions 1) and 2)
\(ab = -3a\)
\(⇔ ab + 3a = 0\)
\(⇔ a(b+3) = 0\)
⇔ \(a = 0\) since \(b > 0\) and \(b ≠ -3.\)

The answer is 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.

Answer: C
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Re: Is a=0?   [#permalink] 01 Jan 2018, 02:10
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