Stevens and Sons Painting won the bid to paint 4 identical houses. Mr. Stevens painted the first house in 5 days. One of his sons painted the second house in 4.5 days, and the other son painted the third house in 4.5 days. If Mr. Stevens and his two sons work together, how long will it take them to paint the last house?

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.