A man can do a piece of work in 10days, but with the help of his son, he can finish it in 5days. In what time can the son do it alone?	Select:  A) 5  B) 8  C) 7  D) 10  E) 15

Rationale: "son ' s 1 day work = 1 / 5 - 1 / 10 = 1 / 10 son alone can do the work in 10 days answer is d"
Answer: D