The simple interest on an amount for 4 years at 5% p.a. is ₹1,800 less than the simple interest on the same amount for 7 years at the same rate. Find the principal amount.

A sum becomes 2215 of itself in 6 years at simple interest. Find the rate of interest per annum.

In how many years will a sum of ₹5,500 amount to ₹6,600 at a simple interest rate of 5% per annum?

A sum of money amounts to ₹16000 in 6 years at simple interest of 25 percent per annum. What will be the total interest for 6 years?