A sum of money at simple interest amounts to Rs. 825 in 3 years and to Rs. 830 in 4 years. The sum is? Choose the correct choice from the following:
A) s . 738  B) s . 638  C) s . 650  D) s . 810  E) s . 735

Rationale: "s . i . for 1 year = ( 830 - 825 ) = rs . 5 s . i . for 3 years = 5 * 3 = rs . 15 principal = ( 825 - 15 ) = rs . 810 . answer : d"
Answer: D