A man can do a piece of work in 7days, but with the help of his son, he can finish it in 3days. In what time can the son do it alone? Choose the correct choice from the following choices
A) 5 B) 4 1/5 C) 7 1/2 D) 6 E) 9 1/2
Son's 1day work = 1/3 - 1/7 = 5/21   Son alone can do the work in 21/5days = 4 1/5 days   Answer is 4 1/5
<result>B</result>
A man can do a piece of work in 7days, but with the help of his son, he can finish it in 3days. In what time can the son do it alone?