This is esay to do when you realize the the speed of mowing is addtive. This is if John mows at 1 lawn / (2/3) hour and Gerge mows at 1 lawn / 1hour, the total speed of mowing is:
1 / (2/3) + 1 = 3/2 + 1 = 5/2
=> 5/2 of lawn per hour
Now, the time is calcualted as the amount of lawn divided by the speed:
1 lawn / (5/2 lawn/hour) = 2/5 hour.
Answer: 2/5 of an hour.
