It would take one machine 4 hours to complete a large production order and another machine 3 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?
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A$$\frac{7}{12}$$
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B$${1}\frac{1}{2}$$
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C$${1}\frac{5}{7}$$
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D$${3}\frac{1}{2}$$
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E7