Sally can clean the house in 6 hours, her father can clean the house in 4 hours, and her younger brother, dennis, can completely mess up the house in 10 hours. if sally and her father clean and dennis plays, how long will it take to clean the house?
The total time it takes for the two of them to clean the house is 5 hrs. Sally and her father can clean the house twice as fast as Denis can mess it up.
