There are infinitely many possible answers. Here's one:
Un = (-10n3 + 75n2 - 167n + 174)/6 for n = 1, 2, 3, ...
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The answer is 15 because the pattern is subtract one, add two
(4*10+11+2*12+13+2*14)/10 = 11.6
It is (8+6+11+12+13)/5 = 10
10 7 11 12 9 13 14 11 15 16 13 17 18 15 19 20...
What would be the next number in this series 15 12 13 10 11 8?