HMMT 十一月 2009 · 冲刺赛 · 第 5 题

英文解析

5. [ 6 ] Tanks has a pile of 5 blue cards and 5 red cards. Every morning, he takes a card and throws itdown a well. What is the probability that the first card he throws down and the last card he throwsdown are the same color?
   Answer: Once he has thrown the first card down the well, there are 9 remaining cards, and only 4
   4 have the same color as the card that was thrown down. Therefore, the probability that the last card 9
   he throws down has the same color is .4
   10329