mkbigmoz wrote:The answer is 2, but I didnt use your formula. I wrote out the possibilities and came up with 4 (7-8, 8-9, 8-7, 9-8). Obviously my answer was wrong, but could you explain why I cant reverse the integers?

In a set (or in a subset), the order of the elements is not important. So, the set {1, 3, 5} is the same set as {3, 1, 5}, or {5, 1, 3}, etc. Since this question asks how many subsets we can choose, the order is not important, and {7, 8} should be considered to be the same as {8, 7} (and this is the reason it's a 'combinations question', and not a 'permutations question').

Since the answer here is just two, writing out all of the possibilities is a perfectly good approach - {7, 8} and {8, 9} are the only answers - though if the question featured a larger set, the approach would not be practical.

That said, while I could imagine this kind of question appearing on a GMAT, I don't believe I've ever seen a real GMAT question that tested counting with this setup (choosing subsets from a set), so understanding this particular question isn't likely to be important on test day.