Marcy and Justin work at the same rate, and together they can paint a house in nine days. They get Ron, who also works at the same rate as Marcy and Justin, to help them paint the house.
How long will it take the three of them to paint the house?
It would take any one person 18 days to paint the house alone. So, each day, each person paints 1/18 of the house (all working at the same rate). When three people are painting they can finish (3 x 1/18) or 1/6 of the house each day. Since 1/6 is painted per day, it takes 6 days to complete the painting.