Brett can plow out all the driveways on his street with his new four-wheel-drive truck in 6 hours. Using a snow blade on a lawn tractor, his neighbor can plow out the same number of driveways in 12 hours. How long would it take them to do the work together?
The truck works twice as fast as a lawn tractor, so is equivalent to 2 of them. Together, they are equivalent to 3 lawn tractors, so can do the job in 1/3 the time of one of them: 4 hours.
