SOLUTION: Derek can wash and was a car in exactly 2 hours. Allison can do the same job in exactly 5 hours. How long will it take both of them working together (at their usual rates) to wash

Question 876901: Derek can wash and was a car in exactly 2 hours. Allison can do the same job in exactly 5 hours. How long will it take both of them working together (at their usual rates) to wash and wax the car.

Answer by LinnW(1048) (Show Source): You can put this solution on YOUR website!
Derek's rate is 1/2 car per hour
Allison's rate is 1/5 car per hour
Set t = time to complete
(1/2 + 1/5)t = 1

multiply each side by 10/7

t = 1 3/7 hours
checking
(1/2 + 1/5)(10/7) = 1
See
http://www.wolframalpha.com/input/?i=%281%2F2+%2B+1%2F5%29%2810%2F7%29
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