The simple interest on a certain sum at 5% per annum for 4 years is Rs. 800. What will be the compound interest on the same sum for the same period at the same rate?

The compound interest is about Rs. 862.

From SI = P x R x T / 100, 800 = P x 5 x 4 / 100, so P = 4000.

Amount = P x (1 + R/100)^T = 4000 x (1.05)^4.

(1.05)^4 = 1.21550625, so amount = 4862.03.

CI = 4862.03 - 4000 = 862 (approx).