题库搜索>问题求解PS-13205
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?
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抽到的为蓝色的可能性为 C2，9
一共的可能性为C2，x
那么C2，9/C2，x=6/11
9*8/2：x（x-1）/2=6/11
72：x（x-1）=6:11
6x（x-1）=72*11
x（x-1）=12*11
所以x=12
考到770不是梦
9*8/n(n-1),带入n算最快……
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2018-10-11 16:48:16
Dulaan
设牌的总数为m张，则 第一次抽中蓝色的几率为9/m，因为不放回，所以第二次抽中蓝色牌的几率为8/m-1 所以，两次都为蓝色牌的几率为 9/m X 8/m-1=6/11 解得m=12
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2018-07-29 09:45:15
发呆的夜之亡魂
主要是分母11太特殊了，所以我就在附近找，结果12没问题
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2018-07-12 22:29:32
已经好几战了
我用了比较笨的但是用时还可以，根据题目可以算出有66种可能，只有12符合条件，嘿嘿。
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2018-04-02 16:22:31
默默哒
n!=n(n-1)(n-2) 设总数=x，设red card为x比较难算。
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2017-10-04 12:35:48
草民不敢
分子简单了，就是C(9,2)=9*8/1*2=36，题目已经给出比例是6/11，所以反推分母应该是66，假设红色扑克是X，分母就是C[(9+X),2]=12*11/1*2=6*11=66. X＝3，扑克总数是9+3=12，选C
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2016-06-26 21:42:43
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