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
Son's 1day work = 1/4 - 1/5 = 1/20   Son alone can do the work in 20days = 20 days   Answer is 20
<result>E</result>
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?