Question 54537
Find the rate at which each of you can mow the lawn:
If you can mow the lawn in 4 hours, then you can mow 1/4 of the lawn in 1 hour.
If your friend can mow the same lawn in 5 hours, then he/she can mow 1/5 of the lawn in 1 hour.
So together, the two of you can mow 1/4 + 1/5 of the lawn in 1 hour.
1/4 + 1/5 = (5+4)/20 = 9/20 of the lawn in 1 hour.
So, it will take you both 20/9 hours to mow the lawn together.
20/9 hours = 2 hrs and 13 1/3 minutes.