Explain the rule of 72? How can it be used in finance applications (see footnote 3.)

³In a shortcut solution to this type of problem known as the Rule of 72, the number 72 is divided by the interest rate to determine the number of years it would take for a sum of money to double. In this case, 72/8% = 9. The Rule of 72 can also be used to determine the interest rate required for a sum of money to double in a given number of years: 72/9 = 8%. The Rule of 72 does not yield exact figures, but it can be used to calculate good approximations.