One possible answer, based on polynomial functions is Un = (8n5 - 140n4 + 920n3 - 2800n2 + 3887n - 1860)/15 for n = 1, 2, 3, ...
27x3+6=87.
3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 plus 3 is 54.
3 plus 3 plus 3 is 9. 3 plus 3 plus 3 plus 3 plus 3 plus 3 is 18.
The answer to your question is twenty four.
15
25
19 is the answer to 3 plus 4 plus 4 plus 3 plus 5
It is 77
12
356,433,300
3 + 3 + 3 + 2 = 11