GMAT Strategy

So I was working with a private GMAT student of mine, and we came across Problem Solving 96 from the 2016 edition of the Official Guide:

If n = 20! + 17, then n is divisible by which of the following?

I. 15
II. 17
III. 19

(A) None
(B) I only
(C) II only
(D) I and II
(E) II and II

I explained to my student why 17 was definitely a factor, but that neither of the other numbers were, so the answer is C.

He said "I know, I got C at first too, and I couldn't prove that 15 or 17 were factors...but my gut told me that one of those other answers was probably true, and that I was missing something, so I chose D."

The lesson here: don't trust your gut. Trust the math. Don't choose an answer that you have no proof for just because it seems right. The GMAT is too complicated of a test for it to fall into any sort of predictable pattern.

On the GMAT Quant, intuition will lead you down the path to a mediocre score. Instead, be a scientist and only trust the hard evidence. Yes, there is always a chance that you will make a mistake in the math, but if you don't understand the underlying mathematical concepts, then you've got a more fundamental problem that needs to be tackled first, before any strategy comes into play.