You did:
8 cars waxed / 3 hours = 2.666 cars waxed/hr
You found how many cars were waxed in an hour.
You should have found how many hours it took to wax one car, because you need to find how long it would take to wax 60 cars.
Instead of 8/3, you do:
3 hours / 8 cars waxed = 0.375 hours/1 car waxed
Now you multiply 0.375 with 60 = 22.5 hours