Jon has $5,000 to invest in a savings account that has interest compounded annually. if he wants his money to double in eight years, what percent must the interest rate be on the account?

log(1 + rate) = {log(total) -log(Principal)} ÷ Years

log(1 + rate) = {log (10,000) - log (5,000)} / 8

log(1 + rate) = {4 - 3.6989700043} / 8

log(1 + rate) = 0.3010299957 / 8

log(1 + rate) = 0.0376287495  

Now we take 10 and raise it to the power of 0.0376287495

10 ^ 0.0376287495 = 1.0905077327

We subtract one:
.0905077327
then multiply it by 100
9.05077327%

Source:
http://www.1728.org/compint2.htm

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