The "Rule of 72" is a shortcut method to answer this question, "How long will it take to double my money if I earn a specified percentage such as 10 percent? Divide 72 by the rate, and the rule gives you an approximation of the years: 72⁄10 5 = 7.2 years. The correct answer is 7.3 years. How accurate is this shortcut for the following rates: 5 percent, 7 percent, 12 percent, and 20 percent?