To understand the rule of 72 formula, you need to divide 72 by the expected annual rate of return. For example, say you invest Rs 1 lakh every year in an investment that earns 8% interest annually. Now if you divide 72 by 8, you will get 9 which gives you the number of years it will take for your money to double. So, your investment will grow to Rs 2 lakh in nine years.