Joe deposits $3500 into a bank account that earns interest compounded continuously. If Joe's money
doubles in 9 years, what interest rate was used?

8%.

Step-by-step explanation:

To find this answer, use the compounding interest rate formula, or an exponential function:

Where A is the amount of money after t years, using P (principal) dollars as the investment and r as the rate.

We are trying to solve for r, so substitute the correct values for the other three variables:

Solving for r, we find that r to the ninth power equals 2.

Simplifying this, r is equal to 2^(1/9), or 1.08~. The interest rate was 8 percent.