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Compound Interest

P is the principal (the money you start with, your first deposit)

r is the annual rate of interest as a decimal (5% means r = 0.05)

n is the number of years you leave it on deposit

A is how much money you've accumulated after n years, including interest.

If the interest is compounded once a year:

A = P(1 + r)n

If the interest is compounded q times a year:

A = P(1 + r/q)nq

Back to the story about how that last formula could apply to cockroaches.

Here's a related formula for present value of a future payment.

And two formulas about annuities: amount an of annuity and the present value of an annuity.