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 The Rule of 72 (Posted on 2005-01-25)
The Rule of 72 is a rule of thumb that states that the number of time periods (such as years) that it takes for a sum to double at compound interest is very nearly 72 divided by the percentage interest rate per period. Thus, for example, it takes (almost exactly) 9 years for a sum to double at 8% interest compounded yearly.

1. Using the rule, find the annual rate of increase for an investment that has quadrupled in 24 years. Compare to the exact value.

2. At what interest rate is the rule exact?

3. Justify the rule using mathematical analysis and a few numerical calculations.

 See The Solution Submitted by Richard Rating: 3.6667 (3 votes)

 Subject Author Date No Subject Charlie 2005-01-25 20:00:23 Further justification Tristan 2005-01-25 19:45:13 re(2): Continuous compounding case Jer 2005-01-25 18:53:51 re: Continuous compounding case John 2005-01-25 18:37:39 Continuous compounding case Jer 2005-01-25 18:08:11 Whole number of years Jer 2005-01-25 17:46:57

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