Rachit invests Rs. 12,000 for a 2-year period at a certain rate of simple interest per annum. Prasad invests Rs. 12,000 for a 2-year period at the same rate of interest per annum as Rachit, but in Prasad's case the interest is compounded annually. Find the rate of interest per annum if Prasad receives Rs. 172.80 more as interest than Rachit at the end of the 2-year period.