[求助]GWD数学！求教

aruora

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

正确答案是C。我对这种概率类的题目超级没思路。。能否请各位说一下对于这种题的基本思路？

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数据充分题（选项省略）

In planning for a trip, Joan estimated both the distance of the trip, in miles, and her average speed, in miles per hour. She accurately divided her estimated distance by her estimated average speed to obtain an estimate for the time, in hours, that the trip would take. Was her estimate within 0.5 hour of the actual time that the trip took?

(1) Joan's estimate for the distance was within 5 miles of the actual distance.
(2) Joan's estimate for her average speed was within 10 miles per hour of her actual average speed.

我选的是C就是两个和起来能推出来。正确答案是E 就是两个和起来也不够充分。

既然有了估算的距离和估算的速度，怎么不能算出估计的时间呢？

谢谢！

sophiaruan

(1)设X个卡，那么红色的有X-9个，随意抽两个是蓝色的概率是：C（29）/C(2X)=6/11,so the X=12

(2)第二个题目很绕，预计的距离-实际的距离<=绝对值5,这得多少个答案啊，预计的平均速度-实际的平均速度<=绝对值10，这得多少个可能啊，个人认为算不出的

bayerner

(9/n)*(8/(n-1))=6/11  得出n(n-1)=12*11, n=12

你带一个数值，比如距离和速度都是1 然后就知道C不对了

如果满足条件 距离的误差必须得按速度的倍数来算   速度的误差同理也要按速度的倍数来算