Gmat Prep Probability

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Hi Please can someone explain this to me? Thanks

[dmateer25]

Two out of the four expressions are chosen at random:

4c2

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

So you have 6 possible ways to choose 2 of the expressions.

Only 1 way meets the requirements: (x+y)(x-y)

So you the probability would be 1/6