A sum of Rs. 12000 is invested at R% simple interest for 3 years then the interest received is Rs. X. If 13/8th of the same sum is invested at 3/4th of the rate of interest for 5 years then the interest received is Rs. (2X + 135). Find the simple interest received after 15 years for the sum Rs. (X + 1680) at the R% interest rate.