prep 数学有道题，我认为答案有异议~麻烦帮忙看下！

简单的豆 · 发表于 2013-1-25 16:14:55

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.
题目翻译：一个盒子中的25个球单独是红色，蓝色，白色，并且有1-10数字画在球上面。如果从盒子里随机选择一个球，这个球是白色或球上面数字是偶数的概率是多少？
仅条件一：球既是白色又有偶数在上面的概率为0. 因为P (either white or even)= P (white)+P(even)- P(both),所以条件一无解。排除A、D。
仅条件二：球是白色的概率减去球上面数字是偶数的概率为0.2。条件二仍无解，排除B。
一二同看：也不充分，无法根据P (either white or even)= P (white)+P(even)- P(both)求出。所以选E
条件二可以求出p(white)+p(even),然后条件一不是告诉了p(both)=0了么？那么就应该选D啊·······答案是不是错了？
能有人帮忙看下么？

还有 prep里面用蓝色画出来的题是什么意思？

thirdwheel · 发表于 2013-1-25 16:32:21

很明显是E啊

简单的豆 · 发表于 2013-1-25 22:22:58

呀····但是我根据条件二求出p(white)+p(even)=10/25+5/25,然后条件一告诉p(both)=0所以我求出的答案是3/5.你能不能帮我看看错哪了呢？谢谢~

简单的豆 · 发表于 2013-1-26 09:59:40

知道自己错哪了·······脑袋堵掉了····

prep 数学有道题，我认为答案有异议~麻烦帮忙看下！

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