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M60-09

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 8228
GMAT 1: 760 Q51 V42
GPA: 3.82

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11 Jun 2018, 03:15
00:00

Difficulty:

35% (medium)

Question Stats:

61% (00:57) correct 39% (01:16) wrong based on 18 sessions

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If $$w^2 x^3 y^4 z^5 < 0$$, is $$xyz > 0$$?

1) $$x < 0$$

2) $$y < 0$$

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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 Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 8228 GMAT 1: 760 Q51 V42 GPA: 3.82 Re M60-09 [#permalink] Show Tags 11 Jun 2018, 03:15 1 Official Solution: 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. Modifying the original condition and question: The original condition $$w^2 x^3 y^4 z^5 < 0$$ is equivalent to $$xz < 0$$ since we can ignore terms with even exponents in this type of inequality (they are always positive). Under the modified condition $$xz < 0$$, the question, 'is $$xyz > 0$$?' is equivalent to 'is $$y < 0$$?', which is the same as condition 2). Since condition 1) tells us nothing about the sign of y, the answer is B. 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$79 for 1 month Online Course"
"Free Resources-30 day online access & Diagnostic Test"
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Joined: 20 Nov 2018
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31 Mar 2019, 08:09
I think this is a high-quality question and I agree with explanation.
Intern
Joined: 23 May 2019
Posts: 3

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24 May 2019, 02:54
I think this is a poor-quality question. I do not understand how the answer to the question is B.
I know that given that the powers are even, the outcome will be positive. But, I do not know how to go from there.
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Joined: 23 Feb 2019
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18 Jul 2019, 03:08
tulullus wrote:
I think this is a poor-quality question. I do not understand how the answer to the question is B.
I know that given that the powers are even, the outcome will be positive. But, I do not know how to go from there.

I will try to explain:

Given the stem, we know that either x or y has to be less than 0, but not BOTH, as if both are less than 0, then the expression would result in >0, which violates what we are given.

So we are trying to solve whether xyz>0. We know given the stem, that x and z have opposite signs, therefore, we are needing to determine if y is greater to or less than 0. We don't know the sign of y, due to its having a positive exponent in the stem (could be positive or negative)

Statement 1: x<0 --> this would imply that z>0, but gives no info on y and is therefore INSUFFICIENT

Statement 2: y<0 --> we already know that x and z have opposite signs and now know that y is negative. Therefore we have (pos*neg*neg) which is >0 OR (neg*neg*pos) which is also > 0 SUFFICIENT
Re: M60-09   [#permalink] 18 Jul 2019, 03:08
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M60-09

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