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GMAT Prep 700's level

by atlantic » Sun Jun 29, 2008 10:04 am
Hi guys,

Got this one after 12 correct in a row. It was my question nr. 20 and I believe this one belongs to the 700's level. I got 49 on Quant. in this Prep.

I'll post OA later.
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Source: — Data Sufficiency |
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by erjamit » Sun Jun 29, 2008 11:07 am
Ans is A.

1) (3y+7)y = x => x is a multiple of y. Sufficient

2) x(x-1) = ny ( x(x-1) is a multiple of y).

x = ny/(x-1)...now it not necessary that x is a multiple of y. Insufficient.

Thus, A

I hope it helps

Amit
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by atlantic » Sun Jun 29, 2008 11:46 am
Yep erjamit, you got it. Answer is A.

For you it was a piece of cake. Great. I got it wrong, my answer was D.

Thanks.
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by beeparoo » Sun Jun 29, 2008 8:32 pm
erjamit wrote:x = ny/(x-1)...now it not necessary that x is a multiple of y. Insufficient.
I feel this part warrants additional explanation. For me, at least!

I got to this point as well, but then considered that
n/(x - 1) can be considered a constant for 'y'.

In this case, I considered x to be a multiple of y. Ack! What is wrong with my conclusion?
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by erjamit » Sun Jun 29, 2008 10:35 pm
n/(x - 1) can be considered a constant for 'y'.

ok look...x and y are integers as given by the stem and we are required to find whether x is a multiple of y.

both n and x-1 are integers, but is it necessary that n/(x-1) is also an integer...it can be it cant be...

I hope you are getting my point...

btw DS is my weak link...n m tryin to strengthen it...

Amit
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by Ian Stewart » Mon Jun 30, 2008 8:25 am
beeparoo wrote:
erjamit wrote:x = ny/(x-1)...now it not necessary that x is a multiple of y. Insufficient.
I feel this part warrants additional explanation. For me, at least!

I got to this point as well, but then considered that
n/(x - 1) can be considered a constant for 'y'.

In this case, I considered x to be a multiple of y. Ack! What is wrong with my conclusion?
Well, I look at the statment y is a divisor of x(x-1), and while there could be many possibilities here, one possibility is certainly that y = x-1. And if x > 2, x is not divisible by x-1; the remainder would always be 1 in that case. Choose any x > 2, let y = x-1, and you'll see why the information is insufficient.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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