超难prep1上的DS题，求解。万分感谢

阳光桂公主 · 发表于 2011-4-4 21:05:17

求助PREP1的DS题，谢谢
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.

bbltxl · 发表于 2011-4-4 21:10:43

选e？

basy · 发表于 2011-4-4 21:10:49

From the question stem we can set up this equation

P(White or Even) = P(White) + P(Even) - P(White and Even)

All we can say from the question is that there are 25 balls. We are not told how many of them are of a particular color or how many are even or how many are odd. (Your assumption of 5/25 is therefore incorrect because we cannot assume that only 5 balls are even).

Statement 1: From this we can tell that P(White and Even) = 0.
Tells us nothing about P(White) or P(Even) or P(White) + P(Even)
Therefore, this statement by itself is insufficient.

Statement 2: From this we are told that P(White) - P(Even) = 0.2
Once again insufficient as what we need is P(White), P(Even) and P(White and Even).

Combining Statements 1 and 2: We only know the value of P(White and Even). Given that P(White) - P(Even) = 0.2 does not tell us anything about P(White), or P(Even) or P(White) + P(Even).

Therefore, the answer is E; Both statements are insufficient.

阳光桂公主 · 发表于 2011-4-4 21:16:06

谢谢，终于弄明白了

bohuei0811 · 发表于 2011-4-5 20:54:10

Where can I download this "prep" software? ... Thanks

pandaforever · 发表于 2011-4-5 21:24:12

LS的同学，去困境去看看吧，有资料可以下载的

超难prep1上的DS题，求解。万分感谢

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