It is a geometric mean sequence.
the 2nd term is 20, the 5th term is 0.16
so the next 3 terms are found by multiplying successive terms by the constant x. 20x^3 = 0.16 which is derived by multiplying 20 by x, three times to get 0.16
So x^3 =.008
x=0.2
The first term is found by saying a1^.2 = 20, so a1 = 100
When n = 1 2 3 4 5
t(n) = 100 , 20, 4, .8, .16