The rule of 72 is a quick method for estimating how long it will take to double your investment, by taking the number 72 and dividing it by the expected rate of return. It’s a good tool that helps you figure out if the stock, bond, or savings account is right for you. It’s also a good program to write to test for and prevent bad input because computers can’t divide by zero. And instead of exiting the program when the user enters invalid input, you can just keep prompting for inputs until you get one that’s valid.

Write a quick calculator that prompts for the rate of return on an investment and calculates how many years it will take to double your investment.

The formula is

where r is the stated rate of return.

Example Output

Constraints

• Don’t allow the user to enter 0.

• Don’t allow non-numeric values.

• Use a loop to trap bad input, so you can ensure that the user enters valid values.

Challenge

• Display a different error message when the user enters 0.