The geometric sequence after the sigma is 125(1/5)^(n-1) so the first four terms are
125, 25, 5, and 1
So A is the sum of the first four terms...
The more common formula for the sum of a geometric sequence is:
s(n)=a(1-r^n)/(1-r), a=initial term, r=common ratio, n=term number
With the more specific infinite sum if r^2<1 as n approaches infinity
s=a/(1-r)