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Advanced Quant Strategy Guide, Workout Set 10 #91 (Pg. 253)

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varumilli
 Post subject: Advanced Quant Strategy Guide, Workout Set 10 #91 (Pg. 253)
 Post Posted: Sat Jan 07, 2012 3:25 am 
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Students


Posts: 1
Hi guys,

Question 91 reads:

If |x| != |y|, xy != 0, x/(x+y) = n, and x/(x-y) = m, then x/y = ?

Can someone explain if it's ok to cross multiply to answer this question?

I tried doing it this way, but the answer I got didn't show up in the answer choices:

x = n(x+y), x = m(x-y)
x = nx + ny, x = mx - my

nx + ny = mx - my
ny + my = mx - nx
y(n + m) = x(m - n)

x/y = (n+m)/(m-n)

The correct answer in the answer choices is x/y = 2nm/(m-n), which I wound up getting by picking numbers and testing. Using the same numbers, however, the answer I came to also works.

Any clarification on this would be much appreciated. Thank you.
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tim
 Post subject: Re: Advanced Quant Strategy Guide, Workout Set 10 #91 (Pg. 253)
 Post Posted: Sun Jan 15, 2012 4:34 pm 
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ManhattanGMAT Staff


Posts: 2183
Location: Southwest Airlines, seat 21C
both of these are correct, because n+m = 2nm in this case using the definitions of n and m. sometimes the algebraic solution gives something different from what's in the answer choices as we see here, and when that happens you need to find a way to equate what you have with one of the answer choices..

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Tim Sanders
Manhattan GMAT Instructor
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