The difference between the interest earned on a certain sum invested under compound interest (compounded annually) and simple interest at the same rate of interest for 2 years is Rs.205. If the rate of interest is 5% p.a., find the sum invested (in Rs.).

A sum becomes 25 times of itself when invested for 48 years at r% simple interest per annum. Find the compound interest earned if Rs. 51,200 is invested at r% per annum when interest is compounded half-yearly, for 1.5 years.

Simple interest on a certain sum at 4% per annum for 2 years is ₹80. What is the compound interest on the same sum for the same period at the same rate?