Time to accumulate a given sum Personal Finance Problem

Manuel Rios wishes to determine how long it will take an initial deposit of $8000 to double.

a. If Manuel earns 12% annual interest on the deposit, how long will it take for him to double his money?

b.How long will it take if he earns only 9% annual interest?

c. How long will it take if he can earn 14% annual interest?

d. Reviewing your findings in parts a, b, and c, indicate what relationship exists between the interest rate and the amount of time it will take Manuel to double his money.