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statement 1+2 gives us x4/2 = x4/(x4+1) here we are equating

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statement 1+2 gives us x4/2 = x4/(x4+1) here we are equating [#permalink] New post 18 Jun 2008, 07:52
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Hi Guys,
This is regarding the post I was going through..
My question is

statement 1+2 gives us
x4/2 = x4/(x4+1)
here we are equating x4+1=2 => x4=1

why dont we cross multiply here which gives us x4=0 or 1?

original post:
Question: Given x1, x2, x3, ...; What is x1?

...(1) Xi = Xi-1/2
e.g. X2 = X1/2, X3 = X2/2
No number to find x1. INSUFF

...(2) x5 = x4/(x4 + 1)
No number to find x1. INSUFF

...(1) + (2);

Now we know the format Xi = Xi-1/2
and X5 = X4/(X4+1)

Clearly, X4 + 1 = 2
X4 = 2-1 = 1

X4 = (X3)/2 = 1; X3 = 2
X3 = (X2)/2 = 2; X2 = 4
..Keep going and you will know X1. Thus, (1) + (2) SUFF.
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Re: DS question [#permalink] New post 18 Jun 2008, 12:54
Hi, shmistry,

In general, the cross-multiplication would be the correct approach to the solution. However, in this particular case, it is explicitly stated that all xi are positive – read the problem carefully (see the link below). So, x4 can’t be zero.

http://www.gmatclub.com/forum/7-t41982
Re: DS question   [#permalink] 18 Jun 2008, 12:54
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