Sometimes their answers are wrong

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muden: Newbie | Next Rank: 10 Posts; Posts: 1; Joined: Wed Mar 19, 2008 2:27 am

Sometimes their answers are wrong

by muden » Fri Mar 28, 2008 4:13 am

Hi everyone,

Here is an example I stumbled across quite recently:

If X^3*Y = 24, what is the value of (X^3*Y^3 - X^2*Y^2)?
(1) X^2*Y^2 = 36.
(2) X^3*Y^2 = 72.

The answer page says the correct one is B. However, the statement(1) is also sufficient, since:

X^2*Y^2 = 36 => X*Y = +-6, but if X^3*Y is positive, X*Y can only be positive.
Therefore, X^2*Y^2(X*Y-1)=36(X*Y-1)=36*5 = 180

Please correct me if I'm wrong.

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Source: Beat The GMAT — Data Sufficiency | Fri Mar 28, 2008 4:13 am

codesnooker: Legendary Member; Posts: 543; Joined: Fri Jan 18, 2008 1:01 am; Thanked: 43 times; GMAT Score:580

by codesnooker » Fri Mar 28, 2008 4:42 am

yes you are correct, statement 1 is also alone sufficient.

I think they forget to frame the part of solution that you have discovered.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

by Stuart@KaplanGMAT » Fri Mar 28, 2008 9:22 am

There are a lot of questionable question sources out "there".

Hence, it's always good to post the source of questions. If you told me your question was from Kaplan or MGMAT, I'd be surprised. If you told me it was from some semi-random question set you downloaded for free of the Net, I wouldn't be shocked at all.