Which number is next in the series 8 13 18 24 39?

There seems to be no logical order to this set of numbers.

* * * * *

Or, you could try 78.

Fit the quartic polynomial

t(n) = (7n4 - 66n3 + 221n2 - 186n + 216)/24 for n = 1, 2, 3, ...