SOLUTION: A father can wash a car in 20 min and his son can wash a car in 5 min. If they togather start to wash a car how much time will be required?

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Question 889110: A father can wash a car in 20 min and his son can wash a car in 5 min. If they togather start to wash a car how much time will be required?
Answer by Edwin McCravy(20056) About Me  (Show Source):
You can put this solution on YOUR website!
A father can wash a car in 20 min and his son can wash a car in 5 min. If they togather start to wash a car how much time will be required?
What??  The father takes 4 times as long as his son?  I don't believe anybody
can wash a car, and do a good job, in 5 minutes.  This problem is either
botched or very unrealistic.  However I'll do it anyway:


The father's washing rate in cars per minute is 1 car per 20 minutes or

%28matrix%281%2C2%2C1%2Ccar%29%29%2F%28matrix%281%2C2%2C20%2Cminutes%29%29

The son's very speedy washing rate in cars per minute is 1 car per 5 minutes
or

%28matrix%281%2C2%2C1%2Ccar%29%29%2F%28matrix%281%2C2%2C5%2Cminutes%29%29.

Suppose the answer is x minutes.

Then their combined washing rate in cars per minute is 1 car per x minutes or

%28matrix%281%2C2%2C1%2Ccar%29%29%2F%28matrix%281%2C2%2Cx%2Cminutes%29%29  

The equation comes from 

%28matrix%283%2C1%2C%0D%0A%0D%0A%22Father%27s%22%2C+washing%2C+rate%29%29%22%22%2B%22%22%28matrix%283%2C1%2C%0D%0A%0D%0A%22Son%27s%22%2C+washing%2C+rate%29%29  %22%22%2B%22%22  %28matrix%284%2C1%2C%0D%0A%0D%0ATheir%2Ccombined%2C+washing%2C+rate%29%29

1%2F20%22%22%2B%22%221%2F5  %22%22=%22%22  1%2Fx

Multiply through by LCD = 20x and solve.  The answer is 
x = 4 minutes.

Edwin