Let's try something messier:

If we invest $400 a month at 8.4%, how much will we have in 40 years?

investment amount = $400

growth factor = $1.007

interest per period = .007

number of periods = 480
 

TRY IT:

Invest $250 a month at 9%.  How much will you have in 20 years?

One more...  and this one we'll do yearly instead of monthly.  (So, we'll have to assume it's compounded yearly.)

If we invest $5000 a year at 10%, how much will we have in 25 years?

investment amount = $5000

growth factor = $1.10

interest per period = .10

number of periods = 25
 

YOUR TURN:

Invest $10,000 a year at 15%.  How much will you have in 35 years?