Question

.

Answer 1

explicit formula for geometric sequence:
\[\huge\rm a_n = a_1 (r)^{n-1}\]
where
n= number of term we have to find
r=common ratio
a_1 = first term
so first of all solve for r

Answer 2

you can pick either 1st,2nd, or 3rd term
when n =2 a_n = 20

OR
to find common ratio you can divide any term by the previous one
\[\rm r =\frac{a_2}{a_1} ~~~~~or~~~\frac{a_3}{a_2}\]