A man can do a piece of work in 7 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone?
Choose one:
A) 5 B) 7 C) 15/2 D) 21/4 E) 18/5
Son's 1 day's work = (1/3)-(1/7) = 4/21   The son alone can do the work in 21/4 days   Answer is 21/4
<result>D</result>
A man can do a piece of work in 7 days, but with the help of his son, he can do it in 3 days. In what time can the son do it alone?