A person deposited ₹15,600 in a fixed deposit at 10% per annum simple interest. After every second year he adds his interest each earned to the principal. The interest at the end of 4 years is :

₹6,864

Principal(P)= 15600

Rate(R) = 10%

Interest(I) of two years =10×2 = 20%

                               =20% of 15600

                               = \(\frac{1}{5}\) × 15600

                               = Rs 3120

so add Interest in Principle after every two years

                         =15600+3120

                         =Rs 18720

P=18720

Interest of two years 10×2=20%

                               20% of 18720

                              \(\frac{1}{5}\) × 18720

                              = Rs 3744

So Total Interest for 4 years = 3120+3744

                                         = Rs 6864