68 : 5 :: 87 : ? Work out the rule and complete the analogy.

The digits are added, and if the total still has two digits its digits are added again, until one figure is left. For 68 that is 6 + 8 = 14 and then 1 + 4 = 5. For 87 it is 8 + 7 = 15 and then 1 + 5 = 6. The trap 15 stops after the first addition, but stopping there for 68 gives 14 and not 5. 1 is the difference of the digits, 8 − 7, which on the first pair gives 2, and 56 is their product, where $6\times 8=48$.