A man can do a piece of work in 5 days, but with the help of his son, he can do it in 4 days. In what time can the son do it alone?
Select
A) 15 B) 17 C) 20 D) 8 E) 9
Son's 1day's work = (1/4)-(1/5) = 1/20   The son alone can do the work in 20 days   Answer is 20
<result>C</result>
A man can do a piece of work in 5 days, but with the help of his son, he can do it in 4 days. In what time can the son do it alone?