A sum of money invested at C.I. amounts to Rs. 800 in 3 years to Rs. 840 in 4 years. The rate of interest per annum is______?

A. 2 1/2 %
B. 4%
C. 5%
D. 6 2/3 %
Explanation:
S.I. on Rs. 800 for 1 year = (840 – 800) = Rs. 40 Rate = (100 * 40)/(800 * 1) = 5%