एक निश्चित राशि पर 4% वार्षिक दर से 2 वर्षों का साधारण ब्याज ₹80 है। उसी राशि पर समान अवधि और समान दर से चक्रवृद्वि ब्याज कितना होगा?

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.

The compound interest on a certain sum of money for 2 years is ₹210, and the simple interest for the same period at the same rate is ₹200. Find the rate of interest per annum.