A sum of money amounts to Rs 9800 after 5 years and Rs. 12005 after 8 years at the same rate of simple interest. The rate of interest per annum is :

A. 5%
B. 8%
C. 12%
D. 15%
S.I. for 3 years = Rs. (12005 – 9800) = Rs. 2205. S.I. for 5 years = Rs. ( 2205/3 X 5 ) = Rs. 3675. So Principal = Rs. (9800 – 3675) = Rs. 6125. Hence, rate = ( 100 X 3675/6125 X 5 X 5 )% = 12%.