A man can do a piece of work in 10days, but with the help of his son, he can finish it in 5days. In what time can the son do it alone? Choose the correct choice from the following answers:
A) 5	B) 8	C) 7	D) 10	E) 15
Son's 1day work = 1/5 - 1/10 = 1/10   Son alone can do the work in 10days   Answer is 10
<result>D</result>
A man can do a piece of work in 10days, but with the help of his son, he can finish it in 5days. In what time can the son do it alone?