since it is looking for the sum
at S3, replace n with 3 , 2, and 1 , solve the equation and add all of answers together
when s = 3 the equation becomes 5(1/3)^3-1
5(1/3)^2 = 5/9
when s = 2 the equation becomes 5(1/3)^2-1
5(1/3)^1 = 5/3
when s = 1 the equation becomes 5(1/3)^1-1
5(1/3)^0 = 5
5/9 + 5/3 + 5 = 65/9
the answer is the 3rd choice
