prep-110

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

请教做题思路，谢谢啦

between 1 and 2 standard deviations below the mean.  --〉X低于平均值，大概低1个标准差到2个标准差之间

(21-x)=6  --- 1个标准差

x=15

(21-x)=12   ---- 2个标准差

X=9

9-15之间

D