A drawer holds 7 black socks and 5 blue socks, all loose and identical in feel. Two socks are pulled out in the dark. What is the probability that they form a matching pair, that is, both of one colour?

Total = $C(12,2)=66$. A matching pair is either both black, $C(7,2)=21$, or both blue, $C(5,2)=10$ — that is 31 pairs, so the probability is 31/66. Trap $35/66=7\times 5/66$ is the MISMATCH probability, and note 31 + 35 = 66. Trap 7/22 = 21/66 counts only the black pairs. Trap 1/2 guesses that a match is as likely as a mismatch; with unequal colour counts it never is.