There really is no way of telling, but my guess is 22.
15
The next number in the sequence 5-7-13-31 is 69.
The nest number in the sequence is 18. Note that the difference between each number and the next number in the sequence follows the simple sequence of 1,2,3,4. Obviously the next in the sequence of increases is 5, so 13+5=18.
If the last number is 1/3 (it should be), the next number is 5 and 1/3
The pattern is plus 7. So the next number used to be 1.
20
12
11
11
11.
15
The next number in the sequence 5-7-13-31 is 69.
21
22
15
17?
The nest number in the sequence is 18. Note that the difference between each number and the next number in the sequence follows the simple sequence of 1,2,3,4. Obviously the next in the sequence of increases is 5, so 13+5=18.