Machine A can complete a job in 12 hours, and Machine B can do the same job at of the rate of Machine A

题目

Machine A can complete a job in 12 hours, and Machine B can do the same job at of the rate of Machine A. If Machine A completes of the job and then Machine B finishes the remaining portion, what is the total amount of time taken by Machine A and Machine B?

选项

18 hours

20 hours

24 hours

28 hours

30 hours

解析

设总工作量为60单位。 - A的效率为5单位/小时，B的效率为1单位/小时。 - A完成的工作量为总工作的\(\frac{3}{4}\)，即\(\frac{3}{4}×60 = 45\)单位，剩余工作量为\(60 - 45 = 15\)单位。 - A完成45单位所需时间：\(45÷5 = 9\)小时。 - B完成剩余15单位所需时间：\(15÷1 = 15\)小时。总耗时为\(9 + 15 = 24\)，答案是（C）

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