If the last number is 1/3 (it should be), the next number is 5 and 1/3
12-13-15 etc.... The sequence is add 1 then add 2
21
35
These are in groups of 4, each in the sequence n, n-2, n-3, 2(n-3). So the series continues after 25: 23, 22, 44. So the number after 25 is 23
8
20
12
13.
The next number in the sequence 5-7-13-31 is 69.
11
11
11.
15
21
22
15
17?