A sum of money lent out at simple interest amounts to Rs. 720 after 2 years and to Rs. 1020 after a further period of 5 years. The sum is

A.  Rs. 500
B.  Rs. 600
C.  Rs. 700
D.  Rs. 710
Ans: B

Explanation: 

S.I. for 5 year = Rs.(1020 - 720)
                = Rs. 300.
S.I. for 2 year = Rs.(300/5×2)
                = Rs. 120.
Principal = Rs. (720 - 120)
                = Rs. 600.