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Kalon and his friend Marna own a chimney sweep service company. Working together, they can clean a chimney in 1 5/7 hours. If it takes Kalon 4 hours to clean a 20-foot chimney by himself, how long does it take Marna to clean the same size chimney by herself?

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In 1 hour Kalon by himself can clean 1/4 of a chimney.
In one hour Kalon and Marna together can clean
[tex]\frac{1}{\frac{12}{7}}=\frac{7}{12}\ of\ a\ chimney[/tex]
Therefore in 1 hour Marna working alone can clean
[tex]\frac{7}{12}-\frac{1}{4}=\frac{1}{3}\ of\ a\ chimney[/tex]
So, it takes Marna 3 hours to clean a chimney, working by herself.

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