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# Is a=0?

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 7999
GMAT 1: 760 Q51 V42
GPA: 3.82

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28 Dec 2017, 00:41
00:00

Difficulty:

35% (medium)

Question Stats:

64% (01:22) correct 36% (00:50) wrong based on 88 sessions

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[GMAT math practice question]

Is $$a=0?$$

1) $$ab=-3a$$
2) $$b>0$$

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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 $79 for 1 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" Math Expert Joined: 02 Aug 2009 Posts: 7948 Re: Is a=0? [#permalink] ### Show Tags 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 _________________ Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 7999 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Is a=0? [#permalink] ### Show Tags 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 _________________ 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$79 for 1 month Online Course"
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02 May 2019, 08:45
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Re: Is a=0?   [#permalink] 02 May 2019, 08:45
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