The infinite sum for a geometric series is S∞ =
a1
1 − r
where a1 is the
first term and r is the common ratio such that |r| < 1. The first term is a1 = 5 and the common
ratio is r =
2
3
. Since the common ratio is less than 1 the infinite sum is
S∞ =
5
1 − 2/3
=
5
1/3
= 15