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

investment amount = $400

At the end of each month (period), we earn 0.7%...
So, each $1.00 will turn into $1.007...
(That's 8.4% divided by 12 ---   .084 / 12 = .007
growth factor = $1.007
interest per period = .007

number of periods =
12 x 40 = 480

One more... and this one we'll do yearly instead of monthly:

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

investment amount = $5000

At the end of each year (period), we earn 10%...
So, each $1.00 will turn into $1.10...
growth factor = $1.10
interest per period = .10

number of periods = 25