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

$$\eqalign{ & {\text{S}}{\text{.I}}{\text{. for 2 years}} \cr & = \frac{{15600 \times 2 \times 10}}{{100}} \cr & = 3120 \cr & {\text{Principal for next 2 years}} \cr & = 15600 + 3120 \cr & = 18720 \cr & {\text{S}}{\text{.I}}{\text{. for next 2 years}} \cr & = \frac{{18720 \times 10 \times 2}}{{100}} \cr & = 3744 \cr & {\text{Total S}}{\text{.I}}{\text{.}} \cr & = 3120 + 3744 \cr & = 6864 \cr} $$