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 a tracker fund, it would take you approximately seven years (72 / 10) to double your money. In other words, if you invested PS5,000 today, let it sit there, and earned a 10 percent return, you'd have PS10,000 in about seven years. And it doubles from there, too. Of course, you could have even more by adding a small amount every month using the power of compounding.