74 : 33 :: 85 : ? Choose the number that completes the analogy.

Each digit is squared and the smaller square is taken from the larger. For 74 that is 49 − 16 = 33; for 85 it is 64 − 25 = 39. The trap 89 adds the squares instead, but 49 + 16 = 65 for the first pair, not 33. 40 is the product of the digits, against 28 the first time. 3 is the plain difference of the digits, and though 8 − 5 = 3 looks neat, the first pair would then have answered 3 rather than 33.