If two of the four expressions x + y, x + 5y, x - y, and 5x - y are

Hi brandmanrocks,

To start, you have to always be sure that you are answering the question that is ASKED (sometimes the question is so specific that there's really only one way to approach it). That having been said, most prompts can be approached in more than one way, so you could certainly approach this prompt in the way that you described. However, you have to make sure to 'adjust' your 'math' to account for how you approached the work.

You can certainly treat this question as a permutation, BUT you then have to keep track of every potential option that 'fits' what is asked for....

(4 choices)(3 choices) = 12 possible options in which the order matters. Of the various 'pairs', there would be TWO that 'fit' what we're looking for:

(X+Y)(X-Y)
(X-Y)(X+Y)

Again, we would have to count that as two options because 'your way' is focused on treating this as a permutation.

2/12 = 1/6 so you end up with the same correct answer as you would if you treated this as a 'Combination' question.

GMAT assassins aren't born, they're made,
Rich