Math question need help?

Mike, Oscar, and Chuck travel together. Oscar and Chuck are good hikers; each walks 2 miles per hour. Mike has a bad foot and drives a small car in which two people can ride, but not three; the car travels 6 miles per hour. The three friends adopted the following scheme: They start together, Chuck rides in the car with Mike and Oscar walks. After a while, Mike drops Chuck who walks on; Mike returns to pick up Oscar, and then Mike and Oscar ride in the car till they overtake Chuck. At this point, they change: Chuck rides and Oscar walks just as they started and the whole procedure is repeated as often as necessary. What is the ratio of the travel time that the car carries only one person to the total time of a single cycle of the scheme?