[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。

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

x=12

MM厉害啊。多谢啦！

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)