He spent 10 minutes *9 cars = <<10*9=90>>90 minutes washing cars
He spent another 15 minutes * 6 cars = <<15*6=90>>90 minutes changing oil.
He spent 2 cars * 30 minutes = <<2*30=60>>60 minutes changing tires.
He spent 90 minutes + 90 minutes + 60 minutes = <<90+90+60=240>>240 minutes working
He spent 240 minutes / 60 minutes = <<240/60=4>>4 hours working
#### 4