posted by cj .
If you deposit money into your account that pays 6.5% interest, how long will it take for you to double your money?
The short-cut method is to use the Rule of 72. That is 72/rate = time needed for investment to double. So, 72/6.5 = 11.08 years
Or you could do in more precisely by compounding an investment by 6.5% until it doubles.