Each of the 25 balls in a certain box is either red, blue, or white and has a # 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 # painted on it is 0. (2) The probability that the ball will be white minus the probability that the ball will have an even # painted on it is 0.2.
The answer given is E. 我理解是：如果知道有多少白球，那么就可以计算了。但是根据（1），没有办法；但是（2）可以算出有多少白球啊。再加上知道 even # 的球有12个，则可以算出概率。为什么答案会是E呢？有哪位可以帮忙解答一下吗？谢谢！！
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发表于 2009-2-23 04:16:00
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