What is the missing number 15 35 23 31 31 - 39?

Answer 1

One possibility would be 27.

I see here on the odd locations a growing series, adding 8 each time:

15 +8 = 23; 23 +8 = 31; 31 + 8 = 39

For the even locations, you could take a diminishing series, subtracting 4 each time:

35 - 4 = 31; 31 - 4 = 27.