A sum of money place at compound interest doubles itself in 7 years. In how many years will it amount to eight times itself?
Choose the correct choice from the following answers:  A) 21  B) 25  C) 31  D) 35  E) 41
100 ---- 200 ---- 7   400 ---- 7   800 ---- 7   ------   21 years   21nswer: 21
<result>A</result>
A sum of money place at compound interest doubles itself in 7 years. In how many years will it amount to eight times itself?