Investors are often interested in knowing how long it takes for a particular investment to double. A simple means for making this determination is the “rule of 70,” which says: The doubling time of an investment with an annual interest rate r% compounded continuously is given by d = 70/r.

For interest rate r, use the formula B = Pe^rt to find the doubling time for r = 4, 6, 9, 10, and 12. In each case, compare the value with the value obtained from the rule of 70.