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.
Probability from GMATprep
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Source: Beat The GMAT — Problem Solving |
