Joe & Bob Service
Two brothers, Joe & Bob, run a house painting service for any resident on call.
Joe is a professional painter and can finish painting a house in 4 hours.
Bob, on the other hand, is not as experienced as his elder brother and needs 6 hours to complete the same task.
Today there are 5 houses on their job list.
How many hours does it take for these two brothers to finish all the painting if they co-operate? (Assume each house requires the same amount of work.)