A man can do a piece of work in 5days, but with the help of his son, he can finish it in 4days. In what time can the son do it alone?  Choices.
A) 5 B) 5 1 / 2 C) 7 1 / 2 D) 6 E) 20

Rationale: "son ' s 1 day work = 1 / 4 - 1 / 5 = 1 / 20 son alone can do the work in 20 days = 20 days answer is e"
Answer: E