2004 AMC 12A 第 7 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

7.

一个游戏按照以下规则使用筹码。每一轮中,筹码最多的玩家给其他每个玩家各一个筹码,并且还把一个筹码放入弃置堆。当某个玩家没有筹码时,游戏结束。玩家 AABBCC 分别以 151514141313 个筹码开始。这个游戏会进行多少轮?

A game is played with tokens according to the following rule. In each round, the player with the most tokens gives one token to each of the other players and also places one token into a discard pile. The game ends when some player runs out of tokens. Players A,A, B,B, and CC start with 15,15, 14,14, and 1313 tokens, respectively. How many rounds will there be in the game?

3636

3737

3838

3939

4040

答案:B
知识点:过程模拟不变量
难度评级:1390
小提示:

追踪完整三轮循环中的筹码数。

Track the token counts over one full cycle of three rounds

大提示:

每三轮每个玩家都恰好少一个筹码,所以找出领先者何时变为 00

Every three rounds each player loses exactly one token, so find when the leader hits 00

解答:

三轮后,玩家 AABBCC 分别有 141413131212 个筹码。之后每三轮都会使每个玩家的筹码数减少一个。

经过 3636 轮后,他们分别有 332211 个筹码;第 3737 轮中,玩家 AA 把自己的三个筹码全部给出并放入弃置堆后归零,游戏结束。

所以正确答案是 B

After three rounds the players A,A, B,B, and CC have 14,14, 13,13, and 1212 tokens, respectively. Every subsequent three rounds reduces each player’s supply by one token.

After 3636 rounds they have 3,3, 2,2, and 11 tokens. In the 3737th round player A,A, who has the most, gives away all three of their tokens and runs out, ending the game.

Thus, the correct answer is B.

第 6 题#6
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