The difference between simple and compound interests compounded annually on a certain sum of money for 2 years at 4% per annum is Re. 1. The sum (in Rs. ) is_________?

A. 625
B. 630
C. 640
D. 650
Explanation:
Let the sum be Rs. x. Then,
[x (1 + 4/100)2 – x] = (676/625 x – x) = 51/625 x
S.I. = (x * 4 * 2)/100 = 2x/25
51x/625 – 2x/25 = 1 or x = 625.