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
用概率的定义去做
假设红色的有x张。
9×8/((9+x)(8+x))=6/11
该式子不要去算，分析一下x等于3或者2
(C)
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