Question

A man can do a peace of work in 5 days but with the help of his son he can finish it in 3 days in what time can the son alone do it ?

Answer 1

man alone is 5 days

with his son is three days

using common sense you will understand that man's one day work + sons one day work = total work by man and son

Answer 2

Men can do the work in 5 days
therefore a man can do the work in 1 day= 1/5

With the help of his son he can finish the work in 3 days
therefore he and his son can finish the work in=1/3

Togetherly = 1/5 + 1/3
=3+5/15
=8/15

Boy alone can do the work = 8/15-1/3
=8-5/15
= 3/15
=1/15

Boy alone can do the work in 1 days = 1÷1/15
=1×15/1
=1×15
=15 days