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标题: L2008/10-4-11+12Games求教,烦请各位解答,谢谢 [打印本页]

作者: 翁羽凌虚    时间: 2012-8-12 09:06
标题: L2008/10-4-11+12Games求教,烦请各位解答,谢谢
While on vocation, S receives several messages from work, each message from one of 3 associates: H J and L. S receives at least one and no more than two messages from each of them. S receives each message on the day it is sent. No more than one message is sent each day. The messages are received in a manner consistent with the following:
The first message is not from L
Both the first and last message are from the same person
Exactly once S receives a message from J on the day after receiving one from H
Of the first 3 messages, exactly one is from J.

Exactly once S receives a message from J on the day after receiving one from H
本道GAMES里面的第一个题如下,我觉得有利于理解这个条件(虽然在这个题的答案的帮助下我还没能理解),贴出来:
Which one of the followling  could be an accurate list of the email messages S reveices, Identified by the person each message is from and listed in the order she receives them?
答案: J L H H J

这个应该怎么理解啊?
1、If S receives 6 email messages, the fifth of which is from L, which one of the following must be true?
A. the first message is from J
B。The second message is from L
C.the 3rd is from H
D. the 4th is from J   答案是D:但是我觉得如果将上述条件理解成---H之后只有一个J 那么H是不能再第一个的,不然两个J 都在它后面,那L 也不能再第一个 那不就剩下J在第一个了么

If S receives 2 messages from L, what is the maximum possible number of messages S receives after L's first message but before L's last message?
A。0   B. 1   C. 2答案是B 但是我觉得:如果要所最大值,那就按留个信息算,那按照这个顺序:
J L H H L J 不是L和L 之间可以有两个么
作者: simonshen    时间: 2012-8-12 15:01
你把题目当中的一个条件理解错了。Exactly once S receives a message from J on the day after receiving one from H这句的意思是“有且仅有一次J的信息排在紧接着收到H信息后的那一天”,而不是H后只能有一个J。注意"the day after"跟 "a day after"是不同的。"the day after"是就在那天之后的一天,而"a day after"可以是那天之后的任何一天。
第一题的话,由于收到了6条信息,同时3个人每人最多发两条信息,由此可推知必然是三个人每个人都发了两条信息,即有2个H,2个J,和2个L要按题目要求排列在1-6的序列里。由于第5个是L,由此可推知第6个必然是H。理由是,首先6不可能是L。因为6如果是L的话则1也必须是L(Both the first and last message are from the same person),但L不能是第1个(The first message is not from L)。同时6也不可能是J。因为如果6是J的话,1也必须是J,由此构成的序列是 1(J) 2() 3() 4() 5(L) 6(J)。但题目说有且仅有一次J排在紧接着H后面的一天,而这个排列不可能让J有机会排在紧接着H后面的那一天。因此推出6是H,而1也必然是H。由此构成的序列是1(H) 2() 3() 4() 5(L) 6(H)。为了满足“有且仅有一次J排在紧接着H后面的一天”这个条件,则必然2是J。根据另一条件,前3条里只有一条信息来自J,由此推出3肯定是L,则剩下的4必然又是J的。完整推出的序列就是1(H) 2(J) 3(L) 4(J) 5(L) 6(H)。比较原题选项,得出D是正确答案。
第二题的话,你给出的排列不能满足“有且仅有一次J排在紧接着H后面的一天”这个条件,估计还是因为你没把题意理解清楚。为了满足这个条件的话两个L中间最多只能排一个空。




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