GMATprep PS question

I'd plug in numbers.

P and Q can be any prime numbers, so let's say P=3 and Q=5. Thus, N=45, which is a multiple of 5, so those numbers are okay to use.

So now, we're going to to plug in our numbers to the answer choices to find the ones that are NOT multiples of 25. Once we only have one choice left, we have the right answer.

So, we plug in P=3 and Q=5 into the answers. This will eliminate A, C, and E. Now we only have to worry about B and D. Let's change our numbers and plug in again.

Now, let's say that P=5 and Q=2, thus making N=50, which is a multiple of 5, so these numbers are okay to use. If we plug these into answer choices B and D, we find that answer choice B comes out to be 4. Since 4 is not a multiple of 25, we can elimate that - only leaving answer choice D, which must be the correct answer.

This is a pretty common question type ("must be") - and I always like to solve them in this manner.

Hope this helps!

Thank you VP_Jim!! That was a great explanation!