A sum of money at simple interest amounts to Rs. 815 in 3 years and to Rs. 854 in 4 years. The sum is:

A. 500
B. 698
C. 780
D. 910
Ans: B

Explanation: 

S.I. for 1 year = Rs. (854 - 815) = Rs. 39.
S.I. for 3 years = Rs. (39 * 3) = Rs. 117.
Principal = Rs. (815 - 117)
          = Rs. 698.