15. I arrived at this conclusion because of the following:
29, 38 The difference is 9 between 29, and 38.
3, 8 The difference is 5 between 3, and 8.
14, 15 The difference is 1 between 14, and 15.
The pattern is 9-4= 5 and 5-4=1
So I have 9, 5, 1, a difference of 4.
