There are infinitely many possible rules. The following rule gives the next number as 13.
Un = (-25n3 + 195n2 - 380n + 228)/6 for n = 1, 2, 3, ...
20
20
20
The answer is: 12.
11 is added to each number in the question in turn, the next number will be 49
20
20
18
20
12
20
20
The answer is: 12.
20
The next number in this sequence is 200.
20? Just a guess
29, assuming it is an algebraically reclusive sequence.