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标题: [讨论]涛涛31-11-30 [打印本页]

作者: singledream 时间: 2006-7-12 03:46
标题: [讨论]涛涛31-11-30

Q30:C

In a stack of cards, 9 cards are blue and the rest are red. If 2 cards are to be chosen at random from the stack without replacement, the probability that the cards chosen will both be blue is 6/11. What is the number of cards in the stack?

A. 10

B. 11

C. 12

D. 15

E. 18

Answer:

答案是C，这题怎么做？？？求高手解答。

作者: kallyli 时间: 2006-7-12 11:20

C(9,2)/C(x,2)=6/11

x=12

作者: singledream 时间: 2006-7-12 14:27

MM厉害啊。多谢啦！

作者: ruopuli 时间: 2007-4-17 13:10

there are another way for it:

suppose red is R, then the probability to chose two blue is:

9/(9+R)*8/(9+R-1)=6/11,it can be gotten that R=3, so the number in stack is 12(9+3)

作者: Amber789 时间: 2008-7-17 03:16
我式子列对了, 可没算出来. 郁闷!

作者: jingle0915 时间: 2010-7-30 16:32
xiexie

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