In one day, Mr. Stevens painted 1/5 of a house.
Each son paints 1/4.5 of a house in one day.
If Mr. Stevens and his two sons work together, the would paint (1/5+1/4.5+1.4.5)=(9/45+10/45+10/45)=29/45 of a house in one day.
It will take them 1/(29/45)=45/29 days to finish painting the house, assuming their work is additive, like no waiting for drying for the second coat, etc.
