7. During the Spring Festival at the university, the bumper car stand has a problem with cars breaking down and needing repair. Mechanics can be hired to do this at a salary of $20 per hour, but they only work in teams. So if one person were hired, he would not work alone, but two or three would work together on the same repair. One mechanic repairs cars in an average of 30 minutes; two mechanics take 20 minutes, and three mechanics take 15 minutes. While these cars are down, the lost income is $40 per hour. Cars tend to break down at a rate of two per hour. How many mechanics should be hired?