PREP--17求解

alicejingli

In a certain board game, a stack of 48 cards, 8 of which represent shares of stock, are shuffled and then placed face down.If the first 2 cards selected do not represent shares of stock, what is the probability that the third card selected will represent a share of stock?
我是这样想的：   2C40 * 1C8/(3C48)
可是算完后没有答案
据说答案是 4/23
谁能帮我分析一下我错在哪里了。。。谢谢

sdcar2010

They ask for "what is the probability that the third card selected will represent a share of stock." So you only focus on the drawing of the third card and do not worry about the chance of all three card's makeup.  Therefore, there are 8 "stock"cards among the 46 cards.  To get one of the eight cards, you chance is 8/46 = 4/23.