The correct answer is 18 years.
To determine how long it will take to double your money if you receive a 4% yield on your investment, we use the “rule of 72”. According to the rule of 72, the amount of time required to double your money equals 72 divided by your rate of return. The rule of 72 helps to determine the approximate length of time it will take for your money to double.
This means, if you invest at 4% return, you will double your money in 18 years. The simple arithmetic is:
Years required to double investment = (72/4)
= 18 years.