快速洗衣
（年龄组：少年组； 难度：简单； 类别：代数）
Beaver Joe has started a new laundry business. He has got three machines: a washer, a dryer and a pressing iron. Every machine is connected through its own timer which provides for half an hour of electricity.
海狸乔开了一家新的洗衣店。他有三台机器：一台洗衣机、一台烘干机和一台熨斗。每台机器都通过各自的定时器连接，定时器提供半小时的用电时间。
So, when a client arrives, he needs 90 minutes for all of the three procedures. And three clients using the machinery consequently need 270 minutes.
所以，当一位客户前来时，他需要 90 分钟来完成全部三项程序。而三位客户依次使用该设备则总共需要 270 分钟。
But now, there are three beavers arriving which are really busy. Each one them has enough clothes for a load of its own. But they agree that they want to finish as quickly as possible.
不过现在，来了三只忙碌的海狸。它们每只都带着满满一担子衣服。但它们一致认为，要尽快把活儿干完。
How many minutes does it take for all three of them to finish their laundry?
他们三个人洗完衣服总共需要多少分钟？
A) 90 minutes（90 分钟）
B) 120 minutes（120 分钟）
C) 150 minutes（150 分钟）
D) 270 minutes（270 分钟）
答案：答案C是正确的。
解析：
beaver 1 needs 90 minutes anyway. During his use of the dryer, beaver 2 starts with washing, and finishes drying at the same time that beaver 1 is finishing with the iron and beaver 3 finishes his washing. So beaver 3 needs two more turns (dryer and iron). This is a total of the 90 minutes for beaver 1 plus the two turns of beaver 3 (60 minutes), which is 150 minutes.
海狸 1 无论如何都需要 90 分钟。在海狸 1 使用烘干机期间，海狸 2 开始洗衣，并且在海狸 1 洗完熨烫结束时，海狸 2 的烘干也完成了，而海狸 3 的洗衣也刚好结束。所以海狸 3 还需要再轮两回（烘干和熨烫）。这总共是海狸 1 的 90 分钟加上海狸 3 的两轮（60 分钟），即 150 分钟。
说明：
CPUs are working in a similar way as the laundry. If every device is used as soon it is free, computation becomes faster. Pipelining is an important idea for constructing CPUs. It’s a cheap way to speed up computation.
中央处理器（CPU）的工作原理与洗衣过程类似。如果每台设备一空闲就马上投入使用，计算速度就会加快。流水线技术是构建 CPU 的一个重要理念。这是一种成本低廉的加速计算的方法。
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