SOLUTION: Each of 2 brothers can wash a car in 1 hr.However their sister can wash the car in 45 mins., if all 3 works together how long will it take to wash the car?

Question 427218: Each of 2 brothers can wash a car in 1 hr.However their sister can wash the car in 45 mins., if all 3 works together how long will it take to wash the car?

Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
Use minutes for all 3
1 car in 60 mins --? 1/60 car/minute
With all three, they do 1/60 + 1/60 + 1/45 car/min
= (3+3+4)/180 car/min = 1/18 car per minute
--> 18 minutes per car
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You can use a shortcut
The 2 brothers together take 30 minutes, since they both do 1/60 per minute
Use 30 & 45 and the product/sum
= (30*45)/(30 + 45) = 1350/75
= 18 minutes

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