It takes 10 minutes to wash a car, 15 minutes to change oil, and 30 minutes to change a set of tires. If mike washes 9 cars, changes the oil on 6 cars, and changes two sets of tires how many hours did he work? Let's be accurate as possible.
He spent 10 minutes * 9 cars = 90 minutes washing cars. He spent another 15 minutes * 6 cars = 90 minutes changing oil. He spent 2 cars * 30 minutes = 60 minutes changing tires. He spent 90 minutes + 90 minutes + 60 minutes = 240 minutes working. He spent 240 minutes / 60 minutes = 4 hours working.
The answer: 4.