[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。

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.

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。