[求助]一道PREP数学，请教各位！

xiaoxique89

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

 

      答案选A，没有读懂题。

chenholan

standard deviation标准差
1 standard deviations from the mean就是mean+standard deviation
2.5 standard deviations from the mean就是mean+2.5standard deviation
不在这个范围的（就是大于+2.5，小于-2.5）就是大于2.5 standard deviations from the mean。

Kanewu

QUOTE:

以下是引用chenholan在2009-11-5 6:48:00的发言：
standard deviation标准差
1 standard deviations from the mean就是mean+standard deviation
2.5 standard deviations from the mean就是mean+2.5standard deviation
不在这个范围的（就是大于+2.5，小于-2.5）就是大于2.5 standard deviations from the mean。

 

agree

poorleafer

sd = 3
2.5 * sd = 7.5
20 + 7.5 = 27.5 and 20 - 7.5 = 12.5, the question is asking for a number that's outside of this range (12.5 - 27.5)....so A is correct.

xiaoxique89

明白了，谢谢各位！