Lindsay can paint $$\frac{1}{x}$$ of a certain room in one hour. If Lindsay and Joseph, working together at their respective rates, can paint the room in one hour, what fraction of the room can Joseph paint in 20 minutes?
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A$$\frac{1}{3x}$$
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B$$\frac{x}{x-3}$$
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C$$\frac{x-1}{3x}$$
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D$$\frac{x}{x-1}$$
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E$$\frac{x-1}{x}$$
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正确答案: C