The simple interest on a certain sum of money at 4% per annum for 4 years is Rs.80 more than the interest on the same sum for 3 years at 5% per annum. Find the sum./एक निश्चित राशि पर 4% प्रति वर्ष की दर से 4 वर्ष के लिए साधारण ब्याज 3 वर्ष के लिए 5% प्रति वर्ष की दर से समान राशि पर ब्याज से 80 रूपये अधिक है। राशि ज्ञात कीजिये ।

Let the principal sum be P. Interest at 4% for 4 years = P × 4 × 4/100 = 0.16P. Interest at 5% for 3 years = P × 5 × 3/100 = 0.15P. The difference is Rs.80, so 0.16P - 0.15P = 80. This gives 0.01P = 80, so P = 8000. Verification: Interest at 4% for 4 years = Rs.1280, at 5% for 3 years = Rs.1200, difference = Rs.80. Options A (Rs.9200), B (Rs.1200), and C (Rs.1280) are incorrect.