A sum of money on compound interest amounts to Rs. 8240 in 2 years and Rs. 9888 in 3 years. The rate of interest is	Choices.
A) 10% B) 25% C) 20% D) 12% E) 30%
Explanation:   If a certain sum of money at compound interest amounts to Rs.x in t1 years and Rs.y in t2 years, then the rate of interest per annum can be given by   R=[(y/x)^1/(t2−t1)−1]×100%   R=[(y/x)^1/(t2−t1)−1]×100=[(9888/8240)^1/(3−2)−1]×100=[(9888/8240)−1]×100   =(1648/8240)×100=(1/5)×100=20%   Answer: Option 20%
<result>C</result>
A sum of money on compound interest amounts to Rs. 8240 in 2 years and Rs. 9888 in 3 years. The rate of interest is