If the sample interest on a sum of money 20% per annum for 2 years is $400, find the compound interest on the same sum for the same period at the same rate?  Choose the correct choice from the following options
A) $ 460
B) $ 510
C) $ 440
D) $ 500
E) $ 550

Rationale: "rate = 20 % time = 2 years s . i . = $ 400 principal = 100 * 400 / 20 * 2 = $ 1000 amount = 1000 ( 1 + 20 / 100 ) ^ 2 = $ 1440 c . i . = 1440 - 1000 = $ 440 answer is c"
Answer: C