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A new crew of painters can paint a small apartment in 12 hours.  An experienced crew can paint the small apartment in 6 hours.  How many hours does it take to paint the apartment when the two crews work together? It takes _____ hours to paint the apartment when the two crews work together

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The new crew works at the rate 1/12 apt/hr.
The old crew works at the rate 1/6 apt/hr.

Let x = hours required to paint the apartment when the two crews work together.

That is, 
(1/12 + 1/6  apt/hr)*(x hr) = (1 apt)
(3/12)x = 1
(1/4)x = 1
x = 4 hours

Answer:  4 hours 

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