求助两道PREP数学题，谢谢牛牛们啦,解释我都看不懂。。。。

crystal10361

110.11364-!-item-!-187;#058&007646

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









106.11122-!-item-!-187;#058&007588

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

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【答案】 A

【思路】 more than 2.5 standard deviations from the mean è

20-(2.5*3)大於所求的valueor所求的value 大於 20+(2.5*3) è

A~E答案中只有A符合è答案A

快乐小龙

两道题的都是说的标准差
第一题，标准差是6，平均数21，求的是between 1 and 2 standard deviations below the mean
1个标准差是6，2个标准差是12
因为是below the mean, 所以21-6=15，21-12=9，所以范围就是在9到15之间，然后你看选项的值只有D）12是落在这个范围内的；

第二题和原理和第一题是一样的, more than 2.5个标准差就是 20-7.5>X>20+7.5
所以只有A在范围里

Dorismiemie

这道题也困惑了我好久~~终于懂了。
题干看上去像是在考标准差，实际上，“between 1 and 2 standard deviations below the mean”的意思是“比平均数小1个到2个标准差的数”，所以求出范围：【21-2*6，21-6】=【9，15】

不知道这样想对不对，希望对你有帮助