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GMAT Prep...How much time did u take to solve this one ??

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GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 26 Sep 2009, 17:08
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 26 Sep 2009, 22:52
You can look at Statement 1 conceptually: when we add z to xy, we get something odd. However, when we add xz to xy, we get something even. So certainly one of z or xz is odd, the other even. Now if xz is different from z, then multiplying by x must have changed z, and that could only happen if x is even and z odd. That's a bit tricky to explain, but I hope that's clear.

Or you can proceed algebraically - notice the similarity between the expression in the question and in Statement 1. We know that xy + xz is even, and xy + z is odd. When you subtract this second expression from the first, you're subtracting an odd from an even, so must get an odd: xy + xz - (xy + z) = xz - z = z(x-1) is odd. Since this is a product, z must be odd, and x-1 must be odd, so x is even. Sufficient.

For Statement 2, all the letters could be odd, so not sufficient.
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 27 Sep 2009, 09:58
IanStewart wrote:
You can look at Statement 1 conceptually: when we add z to xy, we get something odd. However, when we add xz to xy, we get something even. So certainly one of z or xz is odd, the other even. Now if xz is different from z, then multiplying by x must have changed z, and that could only happen if x is even and z odd. That's a bit tricky to explain, but I hope that's clear.

Or you can proceed algebraically - notice the similarity between the expression in the question and in Statement 1. We know that xy + xz is even, and xy + z is odd. When you subtract this second expression from the first, you're subtracting an odd from an even, so must get an odd: xy + xz - (xy + z) = xz - z = z(x-1) is odd. Since this is a product, z must be odd, and x-1 must be odd, so x is even. Sufficient.

For Statement 2, all the letters could be odd, so not sufficient.


Fr St2,
y+xz odd
xy+z odd
=> y+zx+xy+z even
=> y(x+1)+z(x+1) even
=> (y+z)(x+1) event
x+1 can be odd or even means that x can be even or odd, insuff
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 28 Sep 2009, 05:34
It took me 15 seconds.

1) sufficient (x is even)
2) insufficient (try even y, odd x, odd z; even x, odd y, z)

A
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 28 Sep 2009, 09:29
Amazing Maratikus, It took me 2mins
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 29 Sep 2009, 10:48
maratikus wrote:
It took me 15 seconds.

1) sufficient (x is even)
2) insufficient (try even y, odd x, odd z; even x, odd y, z)

A


Can you please explain how you solved the question in 15 sec? Did you pick the numbers and solved in 15 sec or was there any other approach....
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Re: GMAT Prep...How much time did u take to solve this one ?? [#permalink] New post 29 Sep 2009, 19:26
Took me less than a min

if XY+Z is odd
these are the possiblities

X Y Z

E E O
O E O
E O O
O O E

so substituting these values in Option A gives the solution write away as Even.
in B we have 2 diff results so

Answer is A
Re: GMAT Prep...How much time did u take to solve this one ??   [#permalink] 29 Sep 2009, 19:26
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