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

[varumilli]

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.

[tim]

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..