A sum of money at simple interest amounts to Rs. 825 in 3 years and to Rs. 840 in 4 years. The sum is? Select.
A) s . 738 B) s . 638 C) s . 777 D) s . 730 E) s . 735

Rationale: "s . i . for 1 year = ( 841 - 825 ) = rs . 16 s . i . for 3 years = 16 * 3 = rs . 48 principal = ( 825 - 48 ) = rs . 777 . answer : c"
Answer: C