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:

S.I. for 3 years = Rs. (12005 - 9800) = Rs. 2205.

S.I. for 5 years = Rs. $\frac{2205}{3}\times 5$= Rs.3675

Principle = Rs.(9800-3675) = Rs.6125

Hence, Rate =  $\frac{100\times 3675}{6125\times 5}$ =12%