Answer:
about 9 years
Explanation:
You want to know the doubling time for money earning an interest rate of 8%.
Rule of 72
The rule of 72 tells you the doubling time is about ...
72/(interest rate %) = 72/8 = 9 . . . . years
The sum will double in about 9 years.
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Additional comment
The actual period will depend on the way interest is compounded. When compounded annually, the time is ...
log(2)/log(1.08) ≈ 9.01 . . . years
Compounded monthly, the time is ...
log(2)/(12·log(1 +.08/12)) ≈ 8.69 . . . years