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Source: — Data Sufficiency |
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by aleph777 » Wed Jan 12, 2011 2:41 pm
Night reader,

Can this have a solution?

If xy < 0, then either x or y must be < 0.

STATEMENT 1: From (y^2)x > 0 we know y^2 is positive, and x must also be positive in order to be > 0. But since we don't know the value of y, INSUFFICIENT.

STATEMENT 2: From (x^2)y > 0 we know that x^2 is positive, and therefore y must be in order for the phrase to be > 0. But here we don't know the value of x, INSUFFICIENT.

COMBINED: This is where I'm confused... Statement 1 says x is positive. Statement 2 says y is positive... So xy > 0. But that contradicts the initial statement... Am I doing something wrong?
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by anshumishra » Wed Jan 12, 2011 3:18 pm
Night reader wrote:If xy < 0, is y < 0?

(1) (y^2)*x > 0
(2) (x^2)*y > 0
xy < 0, is y<0 ?

(1) (y^2)*x > 0
=> (+ve)*x > 0
=> x > 0
So, xy < 0 => y< 0 --- Sufficient

(2)(x^2)*y > 0
=> (+ve)*y > 0
=> y > 0 ----- Sufficient

So, answer is D. Although as per GMAT style(as x has different signs in the two statements), it is not good.
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by Adam@Knewton » Wed Jan 12, 2011 3:19 pm
aleph777 wrote:Night reader,

Can this have a solution?

If xy < 0, then either x or y must be < 0.

STATEMENT 1: From (y^2)x > 0 we know y^2 is positive, and x must also be positive in order to be > 0. But since we don't know the value of y, INSUFFICIENT.

STATEMENT 2: From (x^2)y > 0 we know that x^2 is positive, and therefore y must be in order for the phrase to be > 0. But here we don't know the value of x, INSUFFICIENT.

COMBINED: This is where I'm confused... Statement 1 says x is positive. Statement 2 says y is positive... So xy > 0. But that contradicts the initial statement... Am I doing something wrong?
Aleph: You are correct that this problem is "broken" because the Statements contradict the informatino given in the Prompt, and therefore could not be a real GMAT question. However, note that it isn't a Value question. If we change the prompt to "If xy > 0," then it makes sense; I'll bet if you re-think your deductions based on what the question is really asking for, you'll see a different answer...

[spoiler]All we need is the sign of y, and because xy>0 (or xy<0, either one works for this), knowing the sign of x tells us the sign of y. Thus, either statement is Sufficient, and the answer is D.[/spoiler]
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by Night reader » Wed Jan 12, 2011 3:28 pm
AdamKnewton wrote:
aleph777 wrote:Night reader,

Can this have a solution?

If xy < 0, then either x or y must be < 0.

STATEMENT 1: From (y^2)x > 0 we know y^2 is positive, and x must also be positive in order to be > 0. But since we don't know the value of y, INSUFFICIENT.

STATEMENT 2: From (x^2)y > 0 we know that x^2 is positive, and therefore y must be in order for the phrase to be > 0. But here we don't know the value of x, INSUFFICIENT.

COMBINED: This is where I'm confused... Statement 1 says x is positive. Statement 2 says y is positive... So xy > 0. But that contradicts the initial statement... Am I doing something wrong?
Aleph: You are correct that this problem is "broken" because the Statements contradict the informatino given in the Prompt, and therefore could not be a real GMAT question. However, note that it isn't a Value question. If we change the prompt to "If xy > 0," then it makes sense; I'll bet if you re-think your deductions based on what the question is really asking for, you'll see a different answer...

[spoiler]All we need is the sign of y, and because xy>0 (or xy<0, either one works for this), knowing the sign of x tells us the sign of y. Thus, either statement is Sufficient, and the answer is D.[/spoiler]
Hi Adam, we are not supposed to post the real GMAT questions here ;)
signs are different- yes, the question is not asking one value... so we are not limited to the one single solution, style might be ok
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by Adam@Knewton » Wed Jan 12, 2011 3:33 pm
Night reader wrote:
AdamKnewton wrote:
aleph777 wrote:Night reader,

Can this have a solution?

If xy < 0, then either x or y must be < 0.

STATEMENT 1: From (y^2)x > 0 we know y^2 is positive, and x must also be positive in order to be > 0. But since we don't know the value of y, INSUFFICIENT.

STATEMENT 2: From (x^2)y > 0 we know that x^2 is positive, and therefore y must be in order for the phrase to be > 0. But here we don't know the value of x, INSUFFICIENT.

COMBINED: This is where I'm confused... Statement 1 says x is positive. Statement 2 says y is positive... So xy > 0. But that contradicts the initial statement... Am I doing something wrong?
Aleph: You are correct that this problem is "broken" because the Statements contradict the informatino given in the Prompt, and therefore could not be a real GMAT question. However, note that it isn't a Value question. If we change the prompt to "If xy > 0," then it makes sense; I'll bet if you re-think your deductions based on what the question is really asking for, you'll see a different answer...

[spoiler]All we need is the sign of y, and because xy>0 (or xy<0, either one works for this), knowing the sign of x tells us the sign of y. Thus, either statement is Sufficient, and the answer is D.[/spoiler]
Hi Adam, we are not supposed to post the real GMAT questions here ;)
signs are different- yes, the question is not asking one value... so we are not limited to the one single solution, style might be ok
Night Reader:

You're right, of course this isn't a real GMAC question, because that would violate our terms of service -- but it is worth noting that, as Aleph pointed out above, this question violates the rules of Data Sufficiency, because Statements (1) and (2) taken together imply that x>0 and that y>0, even though we're told that xy<0. One of the most difficult tasks when tweaking a real question so that it is similar to, but different from, a real GMAC question is making sure that weird inconsistencies don't pop up. In the industry we call this SBDing (it stands for "Same-But-Different"). The only reason it's worth pointing out is that otherwise, it could confuse some test-takers.

Thanks for the interesting question!
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by Night reader » Wed Jan 12, 2011 4:11 pm
the reasons that this DS CAN'T BE GMAT question:
there's only one answer - yes or no. this DS has two answers -yes,no or no,yes :)
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by MAAJ » Thu Jan 13, 2011 5:17 am
Statements contradict eachother... and GMAT only presents TRUE information on each statements, so y cannot be <0 and >0 at the same time. Correct answer: [spoiler]Not a real GMAT question :)[/spoiler]
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