求助prep 的Ps 110中的9--15是如何推算出来的？

julia7718

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

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【答案】 D

【思路】 平均數與標準差的概念

題目的數據: 平均數(mean) :21 hrs  標準差(standard deviation) : 6 hrs

由Pat 上星期看電視的時間在1~2個標準差之下可得知(between 1 and 2 standard deviations below the mean)，時間範圍應該坐落在9hrs~15hrs之間 è 答案D 符合算出來的要求

krusher

低于平均数1个——2个标准差之间，低于一个标准差是21-6=15，低于二个标准差是21-12=9，所以是9hrs~15hrs

julia7718

krusher 发表于 2013-9-7 13:59
低于平均数1个——2个标准差之间，低于一个标准差是21-6=15，低于二个标准差是21-12=9，所以是9hrs~15hrs ...

太感谢了，我把21给忘记了，直接用1和2分别乘以6，得到了，6--12