Sally's painting speed = 1 room / 8 hours
Steve's painting speed = 1 room / 9 hours
Combined painting speed = 1 room / 8 hours + 1 room / 9 hours
=> 1/9 + 1/8 = [8 + 9] / 72 = 17 / 72
So, they paint 17 / 72 parts of room per hour.
=> 1 room / x = 17 room /72 hours
=> x = 1 * 72 hours / 17 room = 4.235 hours.
You can verify how much each one has painted in 4.235hours
Sally: 1/8 * (72/17) = 9/17 parts of a room
Steve: 1/9 * (72/17) = 8/17 parts of a room
Sally + Steve = 9/17 + 8/17 = 17/17 = 1 room
