George does 3/5th of a piece of work in 9 days. He then calls Paul and they finish the work in 4 days. How long would Paul take to do the work by himself ?

George 9 days work = 3/5
George 1 days work = 3/5 * 9
= 1/15.
George and Paul 4 days work = 2/5
George and Paul 1 days work = 2/5 * 4
= 1/10.
Paul 1 days work = George and Paul 1 days work – George 1 days work
= 1/10 – 1/15
= 1/30

So Paul takes 30 days to complete work.