Keyword: Rule of 72

This handy rule provides a speedy method of determining the amount of time required to double your principal at a given rate of compound interest. Simply divide the number 72 by the annual return expressed in percentage form. The answer is the number of years required for the principal amount to double. For example, a security yielding 8% compounded annually, assuming no capital gain or loss, would double your principal in 9 (72÷8) years.

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