It is possible for any number to be the next number since it is always possible (and quite easy) to find a polynomial of order 5 that will fit the above numbers and the selected extra one. And that is without considering non-polynomial functions!
The only polynomial of order 4, though, is
Un = (103n4 - 1242n3 + 5201n2 - 8670n + 4680)/24 for n = 1, 2, 3, ... and occording to that, the next number in the sequence is 213.
20
3, -6, 12, 4, 20, ?
56
10 The sequence is given by xn = 60/n
Those are the factors of the number 60, in descending order. The next factor is 10.
25 is the next number that appears in that sequence.
20
20
3, -6, 12, 4, 20, ?
56
10 The sequence is given by xn = 60/n
next in the sequence: 10,20,30,20
16
Those are the factors of the number 60, in descending order. The next factor is 10.
22
The next number in the sequence will be 56.
92 - 18 = 72 72 - 16 = 56 56 - 14 = 42 42 - 12 = 30 30 - 10 = 20 20 is the next number in the sequence.