A man can do a piece of work in 5 days. He and his son working together can finish it in 3 days. In how many days can the son do it alone?
Number of days the man requires to do a piece of work : 5 days
Let Number of days his son requires to do a piece of work : X days
Number of days required by the man and his son together to do a piece of work : 3 days
Work done by the man in one hour: 
Work done by his son in one hour: 
Work done by the man and his Son together in a one hour : 
Work done by the man and his son together in one hour : 
= 
= 
⇒ 3X + 15 = 5X
⇒ 2X = 15
⇒ X = 
= 
days
Son alone can do the work 
days.
2