关于标准差的问题，求解？

helen_stepic

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

mseandy

选择项肯定有范围的 选那个在9-15小时之间的就对了

Winterfish

以下是引用helen_stepic在2007-11-22 11:52:00的发言：

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

nowaylu82

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~~~~