The rule of 72 says to divide 72 over the interest rate without the percent sign attached to it. This means we divide 72 over 9.6 to get 72/9.6 = 7.5
So the money doubles every 7.5 years. When another 7.5 years rolls around, a total of 2*7.5 = 15 years has gone by. At this point the money is roughly 4 times that of the original amount deposited.
