题目信息
Machine A, operating alone at its constant rate, produces 500 feet of a particular fiber in 2 hours. Machine B, operating alone at its constant rate, produces 500 feet of the same fiber in 3 hours. Machine C, operating alone at its constant rate, produces 500 feet of the same fiber in 5 hours. How many hours will it take machines A, B, and C, operating together at their respective constant rates, to produce 1,000 feet of the fiber?
A:$$\frac{60}{31}$$
B:$$\frac{15}{7}$$
C:$$\frac{25}{11}$$
D:$$\frac{10}{3}$$
E:$$\frac{20}{3}$$
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正确答案：
A:$$\frac{60}{31}$$
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