15.Working alone,R can complete a certain kind of job in 9 h

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15.Working alone,R can complete a certain kind of job in 9 hours.R and S,working together at their respective rates,can complete one of these jobs in 6 hours.In how many hours can S,working alone,complete one of these jobs?
(A) 18(B) 12(C) 9
(D) 6(E) 3

R完成工作用9个小时,R与S共同完成需要6小时,问S需要多少小时?那么1/(1/9+1/S)=6,解得S=18

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