A and B undertake to do a piece of work for $500. A alone can do it in 5days while B alone can do it in 10days. With the help of C, they finish it in 2days. Find the share of C?  Choose the most appropriate option
A) $ 200 B) $ 150 C) $ 300 D) $ 25 E) $ 100

Rationale: "c ' s 1 day work = ( 1 / 2 ) - ( 1 / 5 + 1 / 10 ) = 1 / 5 a : b : c = 1 / 5 : 1 / 10 : 1 / 5 = 2 : 1 : 2 b ' s share = 500 * 2 / 5 = $ 200 answer is a"
Answer: A