Probability from GMATprep

looks like 1/6.


You basically need to pick both x+y and x-y. Those are the only two that will satisfy the stem.

Since we know there is only 1 way for this to happen (when you don't consider order choosing x+y and x-y is the same as choosing x-y and x+y), we need to figure out how many ways you can chose 2 equations (without order being important).

4C2 = 4!/(2!*(4-2)!) = 6

Since there's 1 way it can work, and 6 possibilities, we have 1/6.

Thanks, I have struggled with that one.