which of the following must be true

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Goldfinger2001: Senior | Next Rank: 100 Posts; Posts: 50; Joined: Mon Jan 03, 2011 9:22 am

by Goldfinger2001 » Fri Feb 25, 2011 4:29 am

Hi,

I am confused with this type of question:

Where is answer A wrong? And why is OA E right?

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Source: Beat The GMAT — Quantitative Reasoning | Fri Feb 25, 2011 4:29 am

maihuna: Legendary Member; Posts: 1578; Joined: Sun Dec 28, 2008 1:49 amFollowed by: 9 members; Thanked: 82 times; GMAT Score:720

by maihuna » Fri Feb 25, 2011 8:35 am

solving the two sides : xy+z = xy+xz =>z=xz=>z(x-1)=0=> z=0 or x-1=0=>x=1

Charged up again to beat the beast

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Goldfinger2001: Senior | Next Rank: 100 Posts; Posts: 50; Joined: Mon Jan 03, 2011 9:22 am

by Goldfinger2001 » Fri Feb 25, 2011 8:42 am

Hi Maihuna,

thanks for the answer.

It's logical...sometime I makes things more difficult then they are...

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Brian@VeritasPrep: GMAT Instructor; Posts: 1031; Joined: Thu Jul 03, 2008 1:23 pmFollowed by: 255 members; Location: Malibu, CA; Thanked: 716 times; GMAT Score:750

by Brian@VeritasPrep » Fri Feb 25, 2011 10:43 am

One more point on this one - keep in mind that with a "MUST BE TRUE" question, an answer choice with "and" is significantly harder than an answer choice with "or". "And" means that both conditions MUST BE met, whereas "or" means that one or the other will do.

On this one, they use A as a trap answer because if z = 0, then it doesn't matter what x is, the equation is satisfied. So, mentally, you're looking at z = 0 and saying "yes, that satisfies the equation if x also = 0", but that doesn't HAVE TO BE the case; that's just one situation in which the equation works.

So with that MUST BE TRUE standard, make sure you pay particular attention to that kind of wording like "and" vs. "or". "And" means that two independent things must both be true, and that's a pretty high burden of proof...

Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep

Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.

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Goldfinger2001: Senior | Next Rank: 100 Posts; Posts: 50; Joined: Mon Jan 03, 2011 9:22 am

by Goldfinger2001 » Fri Feb 25, 2011 11:12 am

Hi Brian,

thanks for the advice. Its like you said..I haven't seen a question like this before, checked A and said...well looks right for me

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