23 : 55 :: 34 : ? Select the number that completes the analogy.

The number is added to itself written the other way about. 23 with 32 makes 55; 34 with 43 makes 77. The trap 43 stops at turning the digits round without adding, and on the first pair that would have given 32 rather than 55. 12 is the product of the digits, against 6 for the first pair, and 68 simply doubles 34, where doubling 23 gives 46.