Working simultaneously at their respective constant rates, Machines A and B produce 800 nails in x hours. Working alone at its constant rate, Machine A produces 800 nails in y hours. In terms of x and y, how many hours does it take Machine B, working alone at its constant rate, to produce 800 nails?
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A$$\frac{x}{x+y}$$
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B$$\frac{y}{x+y}$$
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C$$\frac{xy}{x+y}$$
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D$$\frac{xy}{x-y}$$
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E$$\frac{xy}{y-x}$$