If john can mow the lawn in 2/3 of an hour and george can mow the lawn in 1 hour, how long will they take to mow the lawn together? 2/5 of an hour 3/5 of an hour 3/4 of an hour

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.

2/5 of an hour.