What will be the difference between simple and compound interest @20% per annum on a sum of $500 after 2 years? Choose the correct choice from the following answers.
A) $ 10
B) $ 20
C) $ 25
D) $ 30
E) $ 40

Rationale: "s . i . = 500 * 20 * 2 / 100 = $ 200 c . i . = 500 * ( 1 + 20 / 100 ) ^ 2 - 500 = $ 220 difference = 220 - 200 = $ 20 answer is b"
Answer: B