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Is a=0?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 4715
GPA: 3.82

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27 Dec 2017, 23:41
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[GMAT math practice question]

Is $$a=0?$$

1) $$ab=-3a$$
2) $$b>0$$
[Reveal] Spoiler: OA

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Math Expert
Joined: 02 Aug 2009
Posts: 5545

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28 Dec 2017, 05:00
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) $$b>0$$
insuff

combined..
b cannot be -3, so a=0
suff
C
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Absolute modulus :http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372
Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

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01 Jan 2018, 01:10
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 $$b ≠ -3.$$

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: Is a=0?   [#permalink] 01 Jan 2018, 01:10
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