Joey can plow a field alone for in four hours. It takes Luigi five hours to plow the same field alone. If they work together, how long will it take them to plow the the field?
Joey's rate = 1/4
Luigi's rate = 1/5
combined rate = 1/4 + 1/5 = 9/20
time at combined rate = 1/(9/20)
= 20/9
= 2.2222.. hrs
= 2 hours and appr. 13 minutes