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, ...
The next number in the sequence is 20. This is because the sequence is decreasing by 2, then decreasing by 6, then decreasing by 2, and then decreasing by 6. So, the next step would be to decrease by 2 again, resulting in the number 20.
20
12
20
18
20
20
20
The answer is: 12.
20? Just a guess
29, assuming it is an algebraically reclusive sequence.
58