求数学神人解题。。。标准差。。

hhftnt · 发表于 2011-11-9 16:50:58

求大神NN数学达人分析2题....

一组数据平均数=20 标准差=3
which of the folloing values is more than 2.5 standard deviation from the mean..

答案选 12。。。求过程。。。

还有一题DS：
Each of the 25 balls in certain box is ethier red blue or white and has a number from 1 to 10 painted on it.If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even nember painted on it?

(1)The probability that the ball both be white and have an even number painted on it is 0.

(2)The probability that ball will be white minus probability that the ball will have an even number painted on it is 0.2.

hhftnt · 发表于 2011-11-9 18:23:39

人工置顶

降半旗致哀 · 发表于 2011-11-9 18:50:28

一组数平均值为20，标准差3，下面哪个距离平均值超过2.5个标准差，2.5*3=7.5,就是问哪个小于20-7.5=12.5和20+7.5=27.5，12<12.5,因此选它。
E，两个合起来也得不出来，第一个是说同时成立的可能为0，即白球和有偶数在上面两种情况不交叉；第二个说是白球的概率比有偶数在上面的概率多0.2，还需要知晓是白球的概率是多少才能得出结论。

hhftnt · 发表于 2011-11-9 19:01:55

第一题原来是这意思！
我还在用标准发差公式在那算- -~~~
THX。。第二题也想明白了~再次感谢

求数学神人解题。。。标准差。。

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