[讨论]涛涛31-11-30

angelavivi

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。

angelavivi

thanks  a lot !!!

smalllittle

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

x=12