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Please help: Prime #'s and odd?

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by Redhorsep » Fri Jul 15, 2011 11:58 am
Please help, this is from Kaplan:

If x and y are prime numbers, is y (x-3) odd?

1. x>10
2. y<3

My answer: Statement two is sufficient, since y is less than 3 and is a prime number, it must be two. An even number (y) can't get the product of odd number by multiplying with any # because only odd x odd=odd

But this is not the Kaplan answer, what did I do wrong?

Thanks for your help!
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by Ian Stewart » Fri Jul 15, 2011 12:29 pm
Redhorsep wrote:Please help, this is from Kaplan:

If x and y are prime numbers, is y (x-3) odd?

1. x>10
2. y<3

My answer: Statement two is sufficient, since y is less than 3 and is a prime number, it must be two. An even number (y) can't get the product of odd number by multiplying with any # because only odd x odd=odd

But this is not the Kaplan answer, what did I do wrong?

Thanks for your help!
Yes, your logic for Statement 2 here is perfect, and Statement 2 is certainly sufficient, since if y=2, we know that y(x-3) is even.

The answer is not B here, however, because Statement 1 alone is also sufficient. If x > 10 and x is prime, then x must be odd, so x-3 must be even. Thus with Statement 1 alone we also can be certain that y(x-3) is even.

So the answer is D.
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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by shoot4greatness » Fri Jul 15, 2011 2:44 pm
Yeah, I got D. Statement 1 rules out the only even prime number (2) and guarantees that x will be an odd number. Odd - Odd = Even. Y can be any prime ( 2 or 3,5,7 (odd)) and the product will be even. Statement 2 states Y is 2. Any multiple of 2 is even. D is the answer.
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