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(A)
A shepherd had 18 sheep. He kept 1/3 on the right side of the field and the rest on the left side. All but nine sheep were killed by an epidemic. How many sheep did the shepherd have left?
一个牧羊人原来有18只羊，他把其中的1/3 放在田野的右边，余下的留在左边。其中9只羊因流行病死掉了。这个牧羊人留下几只羊?
Answer：Nine.
答案：9只。
(B)
Mr Slow， Mr Medium， Mr Fast， and Mr Speed must cross a rickety rope bridge in 17 minutes. The bridge can carry at most two people at a time. Furthermore， it’s dark， and there is only one flashlight； any single person or pair of people crossing the bridge must have the flashlight with them. (The bridge is too wide for the flashlight to be thrown； it must be carried across.)
Each man walks at a different speed. A pair traveling together must walk at the rate of the slower man. Mr Slow can cross the bridge in at most 10 minutes； Mr Medium can cross in 5 minutes； Mr Fast can cross in 2 minutes； Mr Speed can cross in 1 minute. How do all four men get across the bridge in 17 minutes?
Slow先生，Medium先生，Fast 先生和Speed先生必须在17分钟通过一条摇摆的粗绳桥。桥一次最多能通过两个人。此外，天很黑，而且只有一個手电筒；不管是一个人还是两个人同时过桥都要有手电筒。(桥太宽，手电筒不能从一头抛到另一头；必须有人带它过桥)。
每个人过桥的速度是不同的。如果两个人同时过桥必须以走得慢的人的速度通过。Slow先生要10分钟才能越过桥；Medium先生要5分钟才能越过；Fast先生只要2分钟就能越过；Speed先生只需1分钟就能越过。问：怎样才能让四个人在17分钟里都越过桥?
Answer：
Mr Fast and Mr Speed cross first， taking two minutes. Mr Fast returns with the flashlight， taking two minutes. Mr Slow and Mr Medium cross， taking ten minutes. Mr Speed returns with the flashlight， taking one minute. Mr Fast and Mr Speed cross again， taking two minutes.
答案：Fast先生和Speed先生先通过，用2分钟；Fast先生把手电筒带回去，又用了2分钟；Slow先生和Medium先生同时过桥，用10分钟；Speed先生把手电筒带回去，用1分钟。最后Fast 先生和Speed先生同时通过，用2分钟。总共用时17分钟。
(C)
Crows were about to destroy the entire harvest in my orchard， so I fetched my rifle and took a shot at them. Of course， I didn’t hit any， but half of them flew away. However， ten crows soon came back. Later that day I took another shot at them， and once more half flew away and later ten crows came back. That evening I took yet another shot at them， and again half flew away and ten came back. When I counted the crows I discovered much to my horror—That there were as many as there were to start with. How many were there?
乌鸦将会破坏我果园里的全部收成，因此我拿了步枪去打它们。当然，我一只也没射中，但是它们中的一半飞走了。然而，不久10只又飞回来了。后来我又射了它们一次，于是一半飞走了，然后10只乌鸦又回来了。那天晚上我又射了一次，于是一半飞走了，10只回来了。当我数乌鸦时，令我十分惊讶的是，乌鸦和开始时一样多。那儿有多少只乌鸦?
Answer：There were 20 to start with. Half of them (10) flew away but 10 came back every time.
答案：开始时有20只。它们中的一半(10只)飞走了但是10只很快又回来了。
A shepherd had 18 sheep. He kept 1/3 on the right side of the field and the rest on the left side. All but nine sheep were killed by an epidemic. How many sheep did the shepherd have left?
一个牧羊人原来有18只羊，他把其中的1/3 放在田野的右边，余下的留在左边。其中9只羊因流行病死掉了。这个牧羊人留下几只羊?
Answer：Nine.
答案：9只。
(B)
Mr Slow， Mr Medium， Mr Fast， and Mr Speed must cross a rickety rope bridge in 17 minutes. The bridge can carry at most two people at a time. Furthermore， it’s dark， and there is only one flashlight； any single person or pair of people crossing the bridge must have the flashlight with them. (The bridge is too wide for the flashlight to be thrown； it must be carried across.)
Each man walks at a different speed. A pair traveling together must walk at the rate of the slower man. Mr Slow can cross the bridge in at most 10 minutes； Mr Medium can cross in 5 minutes； Mr Fast can cross in 2 minutes； Mr Speed can cross in 1 minute. How do all four men get across the bridge in 17 minutes?
Slow先生，Medium先生，Fast 先生和Speed先生必须在17分钟通过一条摇摆的粗绳桥。桥一次最多能通过两个人。此外，天很黑，而且只有一個手电筒；不管是一个人还是两个人同时过桥都要有手电筒。(桥太宽，手电筒不能从一头抛到另一头；必须有人带它过桥)。
每个人过桥的速度是不同的。如果两个人同时过桥必须以走得慢的人的速度通过。Slow先生要10分钟才能越过桥；Medium先生要5分钟才能越过；Fast先生只要2分钟就能越过；Speed先生只需1分钟就能越过。问：怎样才能让四个人在17分钟里都越过桥?
Answer：
Mr Fast and Mr Speed cross first， taking two minutes. Mr Fast returns with the flashlight， taking two minutes. Mr Slow and Mr Medium cross， taking ten minutes. Mr Speed returns with the flashlight， taking one minute. Mr Fast and Mr Speed cross again， taking two minutes.
答案：Fast先生和Speed先生先通过，用2分钟；Fast先生把手电筒带回去，又用了2分钟；Slow先生和Medium先生同时过桥，用10分钟；Speed先生把手电筒带回去，用1分钟。最后Fast 先生和Speed先生同时通过，用2分钟。总共用时17分钟。
(C)
Crows were about to destroy the entire harvest in my orchard， so I fetched my rifle and took a shot at them. Of course， I didn’t hit any， but half of them flew away. However， ten crows soon came back. Later that day I took another shot at them， and once more half flew away and later ten crows came back. That evening I took yet another shot at them， and again half flew away and ten came back. When I counted the crows I discovered much to my horror—That there were as many as there were to start with. How many were there?
乌鸦将会破坏我果园里的全部收成，因此我拿了步枪去打它们。当然，我一只也没射中，但是它们中的一半飞走了。然而，不久10只又飞回来了。后来我又射了它们一次，于是一半飞走了，然后10只乌鸦又回来了。那天晚上我又射了一次，于是一半飞走了，10只回来了。当我数乌鸦时，令我十分惊讶的是，乌鸦和开始时一样多。那儿有多少只乌鸦?
Answer：There were 20 to start with. Half of them (10) flew away but 10 came back every time.
答案：开始时有20只。它们中的一半(10只)飞走了但是10只很快又回来了。