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the residents of town x participated in a survey to determine the number of hours per week each resident spend watching tv,the distribution of the results of the survey had a mean of 21 hours and a stand deviation of 6 hours.the number of hours the pat, a resident of town x, watched tv 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 tv last week?

30 , 20, 18, 12,  6

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 是12吗?一倍Std.范围是15(21-6),两倍是9,在1,2倍之间,应该是在9到15之间吧。

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是12

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12

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