Given: The amount payable on maturity of a certain sum which is invested for 5 years at a certain rate percent p. a. is Rs. 9,800 and the amount payable on the same sum invested for 10 years at the same rate is Rs. 12,600. Concept used: Simple Interest, SI = (P × R × T) ÷ 100 where P = Principal amount R = Rate of interest per year T = Time in years Calculation: For the 10-year investment plan, Rs. 9800 would be considered as the principal amount. Since the interest being calculated is simple interest, the interest incurred in 5 years = (12600 - 9800) = Rs. 2800 Hence, interest incurred every year = 2800 ÷ 5 = Rs. 560 Now considering the 5-year investment plan, the principal amount = (9800 - 2800) = Rs. 7000 Let the rate of interest be R%. According to the question, (7000 × R × 1)/100 = 560 ⇒ R = (560 × 100) ÷ 7000 ⇒ R = 8 ∴ The rate of interest is 8% p.a.