Series and Sequence/Application (sum of a geometric series) - Supperannuation

hi

on the last question from my maths text book and i've gotten all previous questions right but this last one.

25. Jenny puts aside $20 at the end of each month for 3 years. How much will she have at the end of the 3 years if the investment earns 8.2% p.a., paid monthly?

(p.a. means per annum so yearly)

answer is : $813.16

btw this chapter is based on using the sum of a geometric series formula.

so ive been using this formula

Sn = __a[(r^n) - 1]__

aaaaaaar - 1

a: being the first amount in the geometric series

r: being the ratio or interest rate

n: being the no. of years or the time period.

unless theres another formula or way of figuring this out because i keep stuffing up something in my working out.

thx all help appreciated.