[求助]请教一概率题和CR

G心未泯

哪位大虾可指点此题如何得出C的答案？我觉得是B

A certain roller coaster has 3 cars, and a passenger is equally likely to ride in any 1 of the 3 cars each time that passenger rides the roller coaster.  If a certain passenger is to ride the roller coaster 3 times, what is the probability that the passenger will ride in each of the 3 cars?

A.      0

B.      1/9

C.      2/9

D.     1/3

E.    1  

还有以下这题CR，答案为何选E，觉得跟气候没什么关系阿

Outbreaks of Rift Valley fever occur irregularly in East Africa, several years apart.  When outbreaks do occur, they kill thousands of cattle.  A livestock vaccine against the disease exists but is rarely used.  It is too expensive for farmers to use routinely, and since it is not effective until a month after vaccination, administering it after an outbreak begins helps very little.  Nevertheless, experts predict that use of the vaccine will increase significantly within the next few years.

 

Which of the following, if true, provides the strongest justification for the experts’ prediction?

 

1. Rift Valley fever is spread by mosquitoes, but each outbreak is so widespread that it is impractical to control it by using insecticides.
2. When an outbreak of Rift Valley fever occurs, unaffected countries often refuse to import livestock from the countries affected by the outbreak.
3. It would take less than a month for producers of the vaccine to adjust their production operations to cope with a large increase in demand.
4. Many cattle farmers in East Africa are nomadic or live in remote villages, and such farmers, who have little access to modern veterinary medicine, are particularly hard hit by outbreaks of Rift Valley fever.
5. Recently published research has shown that certain identifiable climatic conditions are almost invariably followed, within two to five months, by an outbreak of Rift Valley fever.

JIJO

第一题：

总数：每次有3种可能性，所以分母是3的三次方，
分子：即使每次坐一辆，做三次，有P(3,3)的取法，因为我觉得时间上有先后次序之分，A,B,C三辆车全排列

所以3!/27=2/9

不知道解释得对不对。

CR还没看，帮不了你。

snoopydior

还有以下这题CR，答案为何选E，觉得跟气候没什么关系阿

Outbreaks of Rift Valley fever occur irregularly in East Africa, several years apart.  When outbreaks do occur, they kill thousands of cattle.  A livestock vaccine against the disease exists but is rarely used.  It is too expensive for farmers to use routinely, and since it is not effective until a month after vaccination, administering it after an outbreak begins helps very little.  Nevertheless, experts predict that use of the vaccine will increase significantly within the next few years.

 

 

 

Which of the following, if true, provides the strongest justification for the experts’ prediction?

 

 

 

1. Rift Valley fever is spread by mosquitoes, but each outbreak is so widespread that it is impractical to control it by using insecticides.
        
2. When an outbreak of Rift Valley fever occurs, unaffected countries often refuse to import livestock from the countries affected by the outbreak.
        
3. It would take less than a month for producers of the vaccine to adjust their production operations to cope with a large increase in demand.
        
4. Many cattle farmers in East Africa are nomadic or live in remote villages, and such farmers, who have little access to modern veterinary medicine, are particularly hard hit by outbreaks of Rift Valley fever.
        
5. Recently published research has shown that certain identifiable climatic conditions are almost invariably followed, within two to five months, by an outbreak of Rift Valley fever.

还有以下这题CR，答案为何选E，觉得跟气候没什么关系阿

Outbreaks of Rift Valley fever occur irregularly in East Africa, several years apart.  When outbreaks do occur, they kill thousands of cattle.  A livestock vaccine against the disease exists but is rarely used.  It is too expensive for farmers to use routinely, and since it is not effective until a month after vaccination, administering it after an outbreak begins helps very little.  Nevertheless, experts predict that use of the vaccine will increase significantly within the next few years.

 

 

 

Which of the following, if true, provides the strongest justification for the experts’ prediction?

 

 

 

1. Rift Valley fever is spread by mosquitoes, but each outbreak is so widespread that it is impractical to control it by using insecticides.
        
2. When an outbreak of Rift Valley fever occurs, unaffected countries often refuse to import livestock from the countries affected by the outbreak.
        
3. It would take less than a month for producers of the vaccine to adjust their production operations to cope with a large increase in demand.
        
4. Many cattle farmers in East Africa are nomadic or live in remote villages, and such farmers, who have little access to modern veterinary medicine, are particularly hard hit by outbreaks of Rift Valley fever.
        
5. Recently published research has shown that certain identifiable climatic conditions are almost invariably followed, within two to five months, by an outbreak of Rift Valley fever.

ABCD都无关

E的意思是说有可靠的研究表明将有可识别的气候的条件使Rift Valley fever在2-5个月之间爆发

题目中说明疫苗无效的原因是因为一般都有1个月的滞后效应

而E的情况说明这次的疫情将在2-5个月后爆发，即现在使用疫苗不会有来不及的可能-所以就加强了专家关于疫苗会得到大规模应用的结论。

不知道我解释清楚了没~

[此贴子已经被作者于2007-9-25 19:45:41编辑过]

vanshow

我觉着体注意两个限制词：题干中的 irregularly 和选哪项中的invariably。

我觉得把题干模仿og总结成一个因果清晰的句子会帮助做题吧：题干说这种病的爆发没有规律，而且打针很贵，但是农民还是给牲畜打了针。所以解释的选项应该是由于某种气候会必然导致这种病。

PS：个人认为解释型的题不同assumption,经常或有题干意外的意外的东西出现（比如这一题的climate)，但是往往这种选项才是正确的！所以即使是看到了一眼看上去无关的选项，也要耐着性子看完！看懂！

lz加油！

G心未泯

感谢各位的解释，明白了！