几道不会的PREP数学题，请教：）

Each of the 25 balls in a certain box is either red, blue, or white and has a number from 1 to 10 painted on it.  If one ball is to be selected 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 painted on it is 0.

 (1)  The probability that the ball will both be white and have an even number painted on it is 0.

正确答案：E

思路不知道错哪了：问的P是两个事件和 白球的概率P(w)=1/3 10个标号中取得偶数的概率P(e)=1/2

                               (1)可得P（white）*P（even）=0  说明他俩无交集 所以直接就是P(W)+P(E) 可以得出答案

                               (2)P(W)-P(E)=0.2 说明P(E)是P(W)的子集 所以P=P(W)

                               这样的话答案就是D了，我的思路错了吗？不知道错哪了？

2

The sum of positive integers x and y is 77.  What is the value of xy ?
     

 (1) x = y + 1  

 (1) x = y + 1  

 (2) x and y have the same tens digit.

正确答案：D

不明白（2）怎么就可以呢？我选的A，（2）可以是30。37/31,36/32,35...

3

Are at least 10 percent of the people in Country X who are 65 years old or older employed?

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

正确答案：B

（2）中20 percent of the men and 10 percent of the women 不是指of the population 65 years old or older中的嘛？还是这句是插入语，读题的时候可以不考虑

谢谢：）

The sum of positive integers x and y is 77.  What is the value of xy ?
     

 (1) x = y + 1  

 (1) x = y + 1  

 (2) x and y have the same tens digit.

正确答案：D

不明白（2）怎么就可以呢？我选的A，（2）可以是30。37/31,36/32,35...

3

Are at least 10 percent of the people in Country X who are 65 years old or older employed?

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

正确答案：B

（2）中20 percent of the men and 10 percent of the women 不是指of the population 65 years old or older中的嘛？还是这句是插入语，读题的时候可以不考虑

谢谢：）

求救啊~数学很不好，大家帮我看看，谢谢

没人回复吗？再顶

第一题，我们可以让白色球和偶数球同时变大或变小而不改变其差值，这样(2)被满足。而只要没有又是白色又是偶数的球，（1）就被满足了。很明显，答案是E

第二题，那两个十位数相同的数家在一起是77？ 只有38，39。

第三题，那一句可不是插入语。

第二题2，lzmm算术算错了，哈哈

第一题还是没懂，啥意思。。

哦，我晓得第一题什么意思了~

LZMM不能认为PW=1/3,PE=1/2的，题目里并没有说是相同几率的，只是有这么些可能的种类

第三题再请教下

Are at least 10 percent of the people in Country X who are 65 years old or older employed?

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

 (1) In Country X, 11.3 percent of the population is 65 years old or older.

正确答案：B

(1)单独不可以（2）说的是在65 years old or older中有20 percent of the men and 10 percent of the women are employed是吧？然后呢？不知道65 years old or older的人数占的百分比，怎么能求出至少有10%percent of the people in Country X who are 65 years old or older employed

不转了，谢谢解答

你也太晕了吧，就算男的比例再小，总数也超过10percent。我估计你是题看不太懂。题目问的就是关于这些老人，为什么还要去管这些老人占总人口的比例？

第3题是问“是否老人中有超过10%的人在职？”

条件2给出“这些老人中男女在职比例各为20%和10%”