Machine A, working alone at a constant rate, can complete a certain production lot in x hours. Machine B, working alone at a constant rate, can complete 1/5 of the same production lot in y hours. Machines A and B, working together, can complete$$ \frac{1}{2}$$ of the same production lot in z hours. What is the value of y in terms of x and z?
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A$${5x-10z}\over{2xz}$$
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B$${2xz}\over{5x-10z}$$
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C$${5xz}\over{x+z}$$
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D$${xz}\over{x+z}$$
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E$${xz}\over{x+2z}$$
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正确答案: B