求助 狒狒逻辑 70题

tmacking

A low-pressure weather system is approaching Plainville; rainfall results from about 70 percent of such systems in the Plainville
area. Moreover, the current season, spring, is the time of year in which thundershowers, which sometimes result from low-pressure systems, are most likely to occur in Plainville.

Knowing which one of the following, in addition to the information above, would be most useful for determine the probability that Plainville will have a thundershower soon?


A.the percentage of thundershowers in Plainville that occur in the spring
B.the percentage of spring rainfalls in Plainville that are thundershowers
C.the percentage of thundershowers in Plainville that result from low-pressure systems
D.whether low-pressure systems in other areas are associated with rainfall
E.whether Plainville has more or fewer thundershowers each spring than do near by towns

答案是c  
个人怎么觉得是b啊  知道70%的低气压导致降雨  又根据b知道降雨中雷阵雨的概率   相乘不就是结论吗    希望大家给点帮助 小弟完分感谢

supermario5

我觉得如果要得出thundershower的概率，b和c都可以做到，但是题目问的是most useful，那么就要选一个能更好预测到未来是否会下thundershower的指标。
条件：
1.low pressure system is approaching说明P(lps) = 1 即lps一定会发生
2.rainfall results from about 70 percent of such systems in the Plainville area说明P(lps→rainfall) = 0.7
题目问再给一个什么条件可以得出P(thundershower)?
所以有两个方向，一个是知道rainfall导致thundershower的概率，然后和0.7相乘
另一个是知道lps导致thundershower的概率，然后和1相乘
但是实际上假设你是天气预报员，你会选择哪个方向？当然选择lps导致thundershower的概率来计算！因为已经知道lps一定会出现（approaching）那么thundershower的概率则成为唯一决定是否会下雷阵雨的条件。而如果选择lps→rainfall→thundershower的方向则增加了一个不确定因素：是否会rainfall?
所以选择the most useful的话最好是选c不选b

JellyC

whopawo 发表于 2011-5-3 20:16
Moreover, the current season, spring, is the time of year in which thundershowers, which sometimes r ...

啊好像明白了！

sdcar2010

Simple.

The passage says that:
1) A low pressure system is coming soon;
2) Seventy percent of a low pressure system leads to a rainfall;
3) Thundershowers SOMETIMES result from low-pressure systems in spring; which is the season that has the highest chance to have a thundershower.

It asks for the probability to have a thundershower soon.

There are two ways to know the answer:
1) The percentage of thundershowers from a rainfall;
2) The percentage of thundershowers from a low-pressure system IN SPRING.

B) is the answer.
C) is not, since it is not specific to the SPRING season.

微酸硬糖

考虑下全概率公式
thunderstorm 与low pressure有关，即low pressure下rainfall有thunderstorm，其他天气情况下也有thunderstorm的概率
即P（A）=P（A/B1)*P(B1)+P(A/B2)*P(B2)……
所以B是不全面的
C更直接点
这样想好像有点牵强。。。。。

dearsunny

答案说是C，但是feifei解说又是B，希望有人更新一下feifei

我个人觉得是B

-- by 会员 maxawei (2011/7/10 3:41:23)

恩！解说的答案和题目里的答案不一样
我本来觉得是B，但后来想了想觉得是C

选B的时候，我认为rainfall的概率等同于spring rainfall的概率，这个概率乘以Bsping rainfall中thundershower的概率就行了，但仔细想了下，rainfall的概率不能等同于spring rainfall的概率，因为后者只发生在spring,而前者是全年的,，所以我觉得B不对

C：low-pressure sys.导致thundershower的概率。由于第一句就说A low-pressure weather system is approaching Plainville，所以这个概率就等同于the probability that Plainville will have a thundershower soon

不知道这么想对不对~~

vivian2011

我觉得是B。
题干中有个词"soon"，说明与现在这个季节spring有关，B把spring与thundershower联系起来了，C中得thundershower的发生概率可以是全年的，即使知道了这个数据也推不出现在,spring是否很快会有thundershower

pplisiqi

但是问题问的只是probability that Plainville will have a thundershower soon，然后说thundershower sometimes result from low-pressure system~难道不是只要考虑 low-pressure weather system 就行了吗？

maxawei

答案说是C，但是feifei解说又是B，希望有人更新一下feifei

我个人觉得是B

jaze

(B) the percentage ofspring rainfalls in Plainville that are thundershowers

P (thundershowers) = P (rainfall) * P(thundershower/rainfall)



(C) the percentage of thundershowers inPlainville that result from low-pressure systems

read carefully, what we can get from this option is that some thundershowers result from low-pressure systems
we can not get the possibility that low-pressure systems result in thundershowers