A sum of money amounts to Rs.12322.8 after 7 years and Rs.15724.8 after 12 years at the same rate of simple interest. What is the rate of interest per annum? एक धनराशि साधारण ब्याज की दर से 7 वर्षों में 12322.8 रु. हो जाती है और समान ब्याज दर से 12 वर्षों में यह 15724.8 रु. हो जाती है | प्रति वर्ष ब्याज की दर क्या है ?

Interest for 5 years (12-7) = 15724.8 - 12322.8 = Rs. 3402. Simple Interest per year = 3402/5 = Rs. 680.40. Total amount after 7 years = Principal + (7 × 680.40). 12322.8 = P + 4762.80. Principal P = 12322.8 - 4762.80 = Rs. 7560. Rate = (680.40/7560) × 100 = 9%. Verification: P = 7560, after 12 years: 7560 + (12 × 680.40) = 7560 + 8164.80 = 15724.80. ✓