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?
分析A选项
分析B选项
分析C选项
分析D选项
分析E选项
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……
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
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
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。
默默哒
n!=n(n-1)(n-2) 设总数=x,设red card为x比较难算。
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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