Use the rule of 72 to solve. manny has his money in a savings account earning 4.5% interest. how long it will take his money to double?

If Manny's initial money in the savings account is x, we are to calculate for the period in which Manny's money becomes 2x. The calculations are shown 
                                               2x = x (1.045)^n
Cancelling both x from the equation and the computing for value of n will give us an answer of 15.75 years.