A sum of money invested at compound interest to Rs. 800 in 3 years and to Rs 840 in 4 years. The rate on interest per annum is. Choose the correct choice:
A) 4%
B) 5%
C) 6%
D) 7%
E) 8%
S.I. on Rs 800 for 1 year = 40   Rate = (100*40)/(800*1)= 
<gadget id="calculator">(100*40)/(800*1)</gadget>
<output>5</output>
5%   Answer: Option 5%
<result>B</result>
A sum of money invested at compound interest to Rs. 800 in 3 years and to Rs 840 in 4 years. The rate on interest per annum is.