Compound Interest | IMO Special

The compound interest on a sum for 5^(th) year is ₹ 7800 and compound interest for sixth year is ₹ 9048 (interest is compounded annually). What is the rate of interest?

A sum of ₹14000 is lent at compound interest (interest is compounded annually) for 3 years. If the rate of interest is 10%, then what will be the compound interest?