I am actually not solving a financial problem, but it's similar. I'm determining the future number of subscribers to a service if subscribers are being added at a constant rate S and declining month over month at rate C% (commonly called a churn rate).

S = 100 subscribers added per month
C% = 10% (you lose 10% of your subscribers every month)

In a spreadsheet, it's easy:

Month 0, 100 = MO

Month 1, 100 + 0.90 x MO = M1
Month 2, 100 + 0.90 x M1 = M2
Month 3, 100 + 0.90 x M2 = M3