求解两道prep (2/5 考试）

大家帮我再看看这样的一道题吧（

the residents of town X participated in a survay to determine the number of hours per week each  resident spent wanting television ,the distribution of the results of the survay had a mean of 21 hours and a standard deviation of 6hours ,the number of hours that P , a resident of town X ,watched television last week was between 1& 2 stardard deviations blow the mean , which of the following would be the number of hours that P watched the televison last week .

30 .20, 18.12.  6

key is 12 ,but i have no idea about it

each of the 25 balls in a certain box is either red ,blue or white and has a number from 1-10painted on it.if one ball is to be elected at random from the box ,what is the probability that the ball selected will either be white or have an even number painted on it

1) the probability that the ball will both be white and have an even number painte on it is 0

2) the probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2

the residents of town X participated in a survay to determine the number of hours per week each  resident spent wanting television ,the distribution of the results of the survay had a mean of 21 hours and a standard deviation of 6hours ,the number of hours that P , a resident of town X ,watched television last week was between 1& 2 stardard deviations blow the mean , which of the following would be the number of hours that P watched the televison last week .

30 .20, 18.12.  6

平均值是21,方差是6,说有个人P,他上周看电视的时间低于平均值"1到2个方差"之间,1个方差是6,2个方差是12

21-12<p<21-6,所以9<p<15,根据选项,只能选12

each of the 25 balls in a certain box is either red ,blue or white and has a number from 1-10painted on it.if one ball is to be elected at random from the box ,what is the probability that the ball selected will either be white or have an even number painted on it

1) the probability that the ball will both be white and have an even number painte on it is 0

2) the probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2

25个球,有红\蓝\白三种颜色,上面分别写着1-10的数字.问任意取一个球,是白的或是写着偶数的概率

题目实际问的是:白+偶-既白又偶

条件1,既白又偶的概率为0,依然不知道"白+偶",所以不充分

条件2,"白-偶"的概率是0.2,但推不出"白+偶-既白又偶",不充分

两条件一起考虑,既白又偶的概率为0,"白-偶"的概率是0.2,还是推不出"白+偶"的概率

KEY:E

ＭＭ真是Ｎ人啊，不知道ＭＭ什么时候考试呢，你一定能拿Ｎ分拉。

你的思路真的是很清晰的，我经常看你的ＪＪ，也有所进步哦，呵呵，prep从原来的４１到现在的４７啦　，多亏大家的帮忙啊。

帮助大家，提高自己，共同进步！

ＭＭ考试加油加油！等好消息！