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.
56
The number which best completes the sequence below is 20 5 30 6 42 7
The next number in the sequence will be 56.
1 2 2 6 3 12 4 20 5 30 6 42 7 56 8 72 9 90 10
56
56
54
56, 8, 72, 9
56
56
56, 8.
56 (just the next number, not the number that completes the sequence)