2023 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.

一个电子显示屏把当前日期显示为一个 88 位整数:先是 44 位年份,再是 22 位月份,最后是该月内的 22 位日期。例如,今年的植树节显示为 2023042820230428。在 20232023 年中,有多少个日期的 88 位显示中每个数字都出现偶数次?

A digital display shows the current date as an 88-digit integer consisting of a 44-digit year, followed by a 22-digit month, followed by a 22-digit date within the month. For example, Arbor Day this year is displayed as 20230428.20230428. For how many dates in 20232023 will each digit appear an even number of times in the 88-digit display for that date?

55

66

77

88

99

答案:E
知识点:数字奇偶性分类讨论
难度评级:1380
解答:

年份贡献数字 2,0,2,32,0,2,3,所以 22 出现两次,而 0033 各出现一次。为了让每个数字最终出现偶数次,月份和日期的四个数字必须提供奇数个 00、奇数个 33,以及偶数个其他每种数字。

只有四个数字可用,因此月份日期串必须恰好包含一个 00、一个 33,以及由某个数字组成的一对。检查合法月份和日期,得到九个日期:01-1301\text{-}1301-3101\text{-}3102-2302\text{-}2303-1103\text{-}1103-2203\text{-}2210-1310\text{-}1310-3110\text{-}3111-0311\text{-}0311-3011\text{-}30

所以正确答案是 E

The year contributes the digits 2,0,2,3,2,0,2,3, so 22 appears twice while 00 and 33 each appear once. For every digit to end up with an even count, the four digits of the month and day must supply an odd number of 00's, an odd number of 33's, and an even number of every other digit.

With only four digits available, the month-day string must use exactly one 0,0, one 3,3, and a repeated pair of some digit. Checking valid months and days leaves nine dates: 01-13,01\text{-}13, 01-31,01\text{-}31, 02-23,02\text{-}23, 03-11,03\text{-}11, 03-22,03\text{-}22, 10-13,10\text{-}13, 10-31,10\text{-}31, 11-03,11\text{-}03, and 11-30.11\text{-}30.

Thus, the correct answer is E.

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