Select the number that best completes the analogy: 9 : 80 :: 13 : ?

The relation is n : n squared minus 1. As 9 squared is 81 and 81 − 1 = 80, the answer is 13 squared minus 1 = 169 − 1 = 168. The tempting 169 is the plain square, the pattern one step short of the rule; 170 adds 1 instead of subtracting it, and 160 fits nothing.