[求助]一道PREP数学，请教各位！

孙权

   请教一道PREP1数学，请给位指点！

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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

      答案选A，没有读懂题。

isaxi

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。

maxdaddy

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.

孙权

  明白了，谢谢各位！