Compound Interest and the Rule of 72

The compound interest rule is when you have money invested. And they pay interest to you because they use your money. So every year you gain money from interest and each year that passes you gain more money from the interest. And that's how you gain more money by the interest they pay you.

For example, if you were to invest $100 with compounding interest at a rate of 9% per annum, the rule of 72 gives 72/9 = 8 years required for the investment to be worth $200, an exact calculation gives 8.0432 years.