Answer:
the most logical answer is
n^2-1
20th term is then 399
Explanation:
The rule behind the sequence could be anything, but it is very likely that we're supposed to notice that the terms are all squares minus 1
0 = 1*1 - 1
3 = 2*2 - 1
8 = 3*3 - 1
15 = 4*4 - 1
24 = 5*5 - 1
So we expect that the explicit formula is n^2-1 and the 20th term is 399