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
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.
As stated, the sequence is best described as that generated by Un = (-8n5 + 135n4 - 812n3 + 2292n2 - 3047n + 1560)/6 for n = 1, 2, 3, ... There would have been a much simpler solution IF the third number had been 30, but it is not so the above is your answer.
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
356,433,300
12