HMMT 十一月 2009 · 冲刺赛 · 第 5 题

HMMT November 2009 — Guts Round — Problem 5

[ 6 ] Tanks has a pile of 5 blue cards and 5 red cards. Every morning, he takes a card and throws it down a well. What is the probability that the first card he throws down and the last card he throws down are the same color? 1032

解析

[ 6 ] Tanks has a pile of 5 blue cards and 5 red cards. Every morning, he takes a card and throws it down a well. What is the probability that the first card he throws down and the last card he throws down are the same color? 4 Answer: Once he has thrown the first card down the well, there are 9 remaining cards, and only 9 4 have the same color as the card that was thrown down. Therefore, the probability that the last card 4 he throws down has the same color is . 9 1032