Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In finance, the rule of 72, the rule of 70 and the rule of 69 are methods for estimating an investment''s doubling time. The number in the title is divided by the interest percentage per period to obtain the approximate number of periods (usually years) required for doubling. Although scientific calculators and spreadsheet programs have functions to find the accurate doubling time, the rules are useful for mental calculations and when only a basic calculator is available. These rules apply to exponential growth and are therefore used for compound interest as opposed to simple interest calculations. They can also be used for decay to obtain a halving time. The choice of number is mostly a matter of preference, 69 is more accurate for continuous compounding, while 72 works well in common interest situations and is more easily divisible. Felix''s Corollary provides a method of estimating the future value of an annuity using the same principles. There are a number of variations to the rules that improve accuracy.