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[求助]两道PREP的题,有一道标准方差的题看不懂啊!

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楼主
发表于 2008-2-5 01:35:00 | 只看该作者

[求助]两道PREP的题,有一道标准方差的题看不懂啊!

1. of the 60 animals on a certain farm,
            2/3 are either pigs or cows
. how many of the animals are cows?

(1)the farm has more than twice as many cows as it has pigs.

(2)the farm has more than 12 pigs.

答案是c, 但是我觉得应该选a。请问2/3是不是指2/3都是pig或者都是cow? 黄色背景部分是不是指cow的数量大于pig数量的两倍?

2. 调查town X的居民每周花费在电视上的时间,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?

30    20    18     12    6

答案是12。

沙发
发表于 2008-2-5 03:19:00 | 只看该作者

1. be careful about condition a : "more than twice" not twice, Cow>2pig

2. 21-12<p<21-6 , so only 12 satisfies the condition.

板凳
发表于 2008-2-5 04:02:00 | 只看该作者

啊,这两道我也做过,真的要非常小心仔细,第一题常想当然,就错了。More than是大于!那选择范围大了去了.

第二个,是说中值是21,那么一个SD的范围是6,所以2个SD就是12,所以范围在9和15之间。只有12符合。画个图就很明显的。

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