A simple formula can help you estimate the number of years required to double your money. It’s called the rule of 72. You simply divide 72 by the interest rate (without the percent sign). For example, with an interest rate of 4%, your money would double in approximately 72, 4, or 18 years. In (a)–(d), determine the approximate number of years it will take for $1000 to double at the given interest rate.

(a) 3%.

(b) 6%.

(c) 8%.

(d) 12%.

(e) If $120 doubles in approximately 22 years, estimate the rate of interest.