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I put B. I don't understand how (1) alone can be sufficient. Because when you plug in any prime number for x>10 using (1) guidelines, you will get an ODD number for (x-3) however we don't know the value of y, what if its 2 or 3, that can change the yes/no answer to the question. Please help the explanation is not thorough and its been bugging me for so long.

I put B. I don't understand how (1) alone can be sufficient. Because when you plug in any prime number for x>10 using (1) guidelines, you will get an ODD number for (x-3) however we don't know the value of y, what if its 2 or 3, that can change the yes/no answer to the question. Please help the explanation is not thorough and its been bugging me for so long.

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

In order the product of 2 integers to be odd, both must be odd. So, y(x-3) to be odd, y must be any odd prime and x must be the only even prime 2, so that x-3=even-odd=odd. In all other cases given product will be even.

(1) x > 10 --> x is not 2, so the product is even. Sufficient.

Or: x is a prime number more than 10, so it's odd --> x-3=odd-odd=even --> y(x-3)=y*even=even.

(2) y < 3 --> the only prime less than 3 is 2, so y(x-3)=even*(x-3)=even. Sufficient.

I put B. I don't understand how (1) alone can be sufficient. Because when you plug in any prime number for x>10 using (1) guidelines, you will get an ODD number for (x-3) however we don't know the value of y, what if its 2 or 3, that can change the yes/no answer to the question. Please help the explanation is not thorough and its been bugging me for so long.

Take a look at the question is y(x-3) odd?

Now, x,y both primes right? So x-3 will always be even except when x = 2, only even prime

(1) x>10 so x is not 2 Suff

(2) y<3 so 'y' has to be 2 cause is the smallest prime number

Remember negatives can't be prime numbers 1 is of course not prime either

Hence D here

Cheers! J

gmatclubot

Re: If x and y are prime numbers, is y(x-3) odd?
[#permalink]
07 Jan 2014, 09:24