78.
Fit the quartic polynomial
t(n) = (7n4 - 66n3 + 221n2 - 186n + 216)/24 for n = 1, 2, 3, ...
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24, they are going up in 3's, 13, 16, 19 and 18, 21 so next number is 24.
6 is.
Every next number is increased by 5. The next number in the sequence is 23.
18
The next number in the sequence is 18. It's +1, +2, +3, +4, ....