[求助]prep破解版里有关standard deviation的几道题

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If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than

2.5 standard deviations from the mean?

(A) 12.0

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5

答案A

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television.  The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours.  The number of hours that Pat, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean.  Which of the following could be the number of hours that Pat watched television last week?

(A) 30

(B) 20

(C) 18

(D) 12

(E) 6

答案D

For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean.  What was the mean score for the examination?

(A) 74

(B) 76

(C) 78

(D) 80

(E) 82

答案A

能否简单说下思路，谢谢~~

在网上找一下标准差的定义不就明白了吗，

不过这些题似乎不理解标准差也可以做，你就把它看成是一个数就行了。

我知道标准差定义啊，但以定义理解做的话根本没法下手么

能不能请简单解释一下思路哈，谢谢~~

以第一道题为例，

2.5 standard deviations 就是说2.5倍标准差， 2.5 standard deviations from the mean 就是离mean2.5倍standard deviations，这道题只有12离mean 20差8，大于2.5倍标准差7.5,so...

就是不要管定义，把它当一个数