The sum of money at compound interest amounts to thrice itself in 5 years. In how many years will it be 21 times itself?  Choose the correct choice:
A) 18 years  B) 9 years  C) 12 years  D) 16 years  E) 30 years
100 ---- 300 --- 5   900 --- 5   1200---5   1500----5   1800---5   2100---5   6 years   Answer: 30 years
<result>E</result>
The sum of money at compound interest amounts to thrice itself in 5 years. In how many years will it be 21 times itself?