Prime Number Question

by **Hei** Tue Oct 09, 2007 3:46 am

Source: a GMAT preparation book from China - no English name

[DELETED by Stacey]

Hi, guys - sorry, I heard from our legal department and we can't accept sources without a clear citation that we can trace back to the legal owner. Apparently there are some products coming out of China that are illegally distributing copyrighted material from other sources. So we have to delete the question. :(

by **Hei** Tue Oct 09, 2007 3:47 am

oops, E should be "1 and 3"

by **RonPurewal** Fri Oct 12, 2007 5:08 am

x = 35 and y = 21 kills all three choices, so it's A.

Of course, these are not random guesses (random guessing is pretty much ineffectual on this question). To come up with these, note that 3x must have exactly ONE MORE PRIME FACTOR than does y (so that the quotient leaves that one factor). It doesn't matter what that prime factor is. So, if we let x = 5 times 7 and y = 3 times 7, then the quotient is (3 x 5 x 7) / (3 x 7), leaving 5.

by **Guest** Sat Oct 13, 2007 2:19 pm

RPurewal Wrote:x = 35 and y = 21 kills all three choices, so it's A.

Of course, these are not random guesses (random guessing is pretty much ineffectual on this question). To come up with these, note that 3x must have exactly ONE MORE PRIME FACTOR than does y (so that the quotient leaves that one factor). It doesn't matter what that prime factor is. So, if we let x = 5 times 7 and y = 3 times 7, then the quotient is (3 x 5 x 7) / (3 x 7), leaving 5.

If x=y, then 3x/y = 3 which is a prime integer greater than 2. So answer is B (Choice 1 only).

by **StaceyKoprince** Thu Oct 18, 2007 5:13 pm

The question asks which of the following MUST be true, not which of the following COULD be true. Roman numeral #1 is not true all the time (Ron gave one example of when it's not true).