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.650
B.690
C.698
D.700
Answer: C

Explanation: 

S.I. for 1 year = Rs. (854 – 815) = Rs. 39
S.I. for 3 years = Rs. (39 x 3) = Rs. 117
Principle = Rs. (815 – 117) = Rs. 698