Eg: If you have 2.5 lakhs today and you need 5lakh in 5 years. Just divide the number 72 by 5, the answer is 14.41%. Thus you need a type of investment avenue, where you earn at least 14.41% p.a. as rate of interest/returns to double your investment amount in 5 years.
This 'Rule 72' helps you to understand about inflation also. It helps you to calculate the amount of time it will take for inflation to make the real value of money half. Let's say present inflation is 5.5%. When you divide 72 by 5.5% the answer is 13.09 years. That is to say, if you have 1 lakh in your kitty today, it would take around 13.09 years for the value of the money to be halved..