A man can do a piece of work in 10days, but with the help of his son, he can do it in 4 days. In what time can the son do it alone?
Select the correct option
A) 13 / 2 B) 17 / 3 C) 15 / 2 D) 20 / 3 E) 9

Rationale: "son ' s 1 day ' s work = ( 1 / 4 ) - ( 1 / 10 ) = 3 / 20 the son alone can do the work in 20 / 3 days answer is d"
Answer: D