The Rule of 72 is a fast trick you can do to figure out how long it will take to double your money. Here’s how it works: Divide the number 72 by the return rate you’re getting, and you’ll have the number of years you must invest in order to double your money. (For the math geeks among us, here’s the equation: 72 ÷ return rate = number of years.) For example, if you’re getting a 10 percent return rate from an index fund, it would take you a little more than seven years (72 divided by 10) to double
