### Formula

T

total ending balance

P

periodic payment amount

N

number of payments

T

total ending balance

P

periodic payment amount

N

number of payments

In finance, the rule of 72, the rule of 70 and the rule of 69 are methods for estimating an investment's doubling time. Felix's Corollary provides a method of approximating the future value of an annuity (a series of regular payments), using the same principles as the Rule of 72.

P

N

T

Precision

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