वार्षिक रूप से संयोजित होने वाली चक्रवृद्धि ब्याज की किस वार्षिक दर पर, कोई धनराशि 4 वर्षों में स्वयं का 625256 गुना हो जाएगी?

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?