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
Pick one
A) 10%  B) 25%  C) 20%  D) 12%  E) 24%
Explanation: Let the sum be P and rate of interest be R% per annum.   Amount after 2 year = 8240   P(1+R/100)T = 8240   P(1+R/100)2 = 8240 --- ( 1)   Amount after 3 year = 9888   P(1+R/100)T = 9888   P(1+R/100)3 = 9888 --- (2)   (2) ÷ (1) =&gt; [P(1 + R/100)3]/[P(1+R/100)2 = 9888/8240   1+R100 = 9888/8240   R/100 = (9888/8240 − 1 )= 1648/8240 = 15   R = 100/5= 
<gadget id="calculator">100/5</gadget>
<output>20</output>
20%   Solution 2   ---------------------------------------------------------------------------------------   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)<sup>1/(t2−t1)</sup> − 1] × 100%   R = [(y/x)<sup>1/(t2−t1)</sup> − 1] × 100 = [(9888/8240)<sup>1/(3−2)</sup> − 1] × 100 = [(9888/8240) − 1] × 100   = (1648/8240) × 100 = (15) × 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