If 1 + 4 = 9, 2 + 8 = 18, 3 + 6 = 15 then 7 + 8 =?

The given logic is as follows,
Sum of the digits + second digit = Answer.
i.e., 1 +4 = 5 + 4 = 9
2 + 8 = 10 + 8 = 18
Similarly, 7 + 8 = 15 + 8 = 23.