Prep-DS-1-158 概率 没有思路？？？

Molybdenum

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.

 

(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.

 

请教 思路没有概念？

罗杰Liu

答案是不是B？

（1）只能推出white ball上的数字非even，但是不知道white ball的数量，所以无法求

（2）ball have even number的概率是0.5，所以取出white ball的概率是0.5+0.2=0.7，

推出white ball有7个。因此可以单独求出

karen_gao

楼上的怎么断定“ball have even number的概率是0.5”？题中只说这25个球标有1-10中的数字，并没有说标有偶数和奇数的具体数值，而且球数25是奇数，也不可能奇偶各一半。所以我认为答案应该是E

waterpoo

25个ball,不是10

bastencools

求P(WUE)---call it X
P(WUE)=P(W)+P(E)-P(WE)=P(W)+P(E)=X
It is known that P(W)-P(E)=0.2, therefore, 2P(W)=X+0.2----X=2P(W)-0.2, 无法得知取出白球的概率, 所以解不出.
进一步分析, 可以得知, 白球范围在[10,23]之间, 双数球范围在[5,18]之间, 但是知道这些也没什么用.
我的解法, 不敢说就是对的

罗杰Liu

我的思路是有问题，看题太匆忙了，误导lz了

答案应该还是E