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关于标准差的问题,求解?

the residents of Town X participated in a survay to determin the number of hours per week each resident spend 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 deviation below the mean. 问PAT 看了 多少小时的电视

此题怎么求解?答案是12

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选择项肯定有范围的 选那个在9-15小时之间的就对了

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QUOTE:

the residents of Town X participated in a survay to determin the number of hours per week each resident spend 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 deviation below the mean. 问PAT 看了 多少小时的电视

此题怎么求解?答案是12

是平均多少小时吧? [(21-6)+(21-2x6)]/2=12

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还是不太明白。。。这是我的弱项,各位xdjm能不能解释一下啊?究竟标准方差应该怎么用啊?谢谢了~~~~

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up,ding !~~~~~~~~

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