If the simple interest on a certain principal at 5% per annum for 5 years is Rs. 900 more than the simple interest on the same principal for 4 years at 4% per annum, find the principal amount.

The principal is Rs. 10,000.

Use SI = P x R x T / 100 for each case.

SI at 5% for 5 years = P x 25 / 100 = 0.25P.

SI at 4% for 4 years = P x 16 / 100 = 0.16P.

0.25P - 0.16P = 0.09P = 900, so P = 900 / 0.09 = Rs. 10,000.