An amount at compound interest sums to Rs.17640/- in 2 years and to Rs.21168/- in 3 years at the same rate of interest. Find the rate percentage? Choose one:  A) 5 %  B) 7 %  C) 9 %  D) 11 %  E) 20 %

Rationale: "explanation : the difference of two successive amounts must be the simple interest in 1 year on the lower amount of money . s . i = 21168 / - - 17640 / - = rs . 3528 / - rate of interest = ( 3528 / 17640 ) × ( 100 / 1 ) = > 20 % answer : option e"
Answer: E