A sum was due for 3 years at 15% rate of simple interest. Which was paid back with three equal instalments of Rs. 1200. Find the due sum?

Let principal = P. Each instalment of Rs. 1200 is paid at end of years 1, 2, 3. Interest for first payment: 2 years on P, second payment: 1 year on remaining amount, third: 0 years. Total amount with interest: P + P×0.15×3 = 1.45P. This equals present value of instalments: 1200/(1.15) + 1200/(1.15²) + 1200/(1.15³) + simple interest corrections. Using equal instalment formula for SI: Each instalment includes principal portion plus interest. P = (1200×3)/(1 + 0.15×2) = 3600/1.3 ≈ 2769, which doesn't match. Actually for SI equal instalments: P = [E×n]/[1 + r(n-1)/2] where E is instalment. P = [1200×3]/[1 + 0.15×1] = 3600/1.15 ≈ 3130. Using reverse calculation: 1200×3 = 3600 total paid, interest = P×0.15×3 = 0.45P, so P + 0.45P = 3600 doesn't work due to timing. Correct formula: Sum of present values at SI rate gives P ≈ 4140.