450 रुपये की मूल राशि पर 4.5% वार्षिक साधारण ब्याज की दर से 81 रुपये का ब्याज प्राप्त करने में कितना समय लगेगा?

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?