Maths Olympiad Prep

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, 2022

Number theory Difficulty 5.0 AIME Prove it Japan

Find the smallest four-digit integer divisible by three and greater than 20222022 such that there are exactly two types of numbers that appear in the digits.

Solution

We call four-digit integers good if their digits consist of exactly two types of numbers.
First, the upper two digits of integers in [2000,2099][2000, 2099] are 2020. So the only good integers in the range are 20002000, 20022002, 20202020, and 20222022. But all of them are less than or equal to 20222022.
Second, the upper two digits of integers in [2100,2199][2100, 2199] are 2121. So the only good integers in the range are 21112111, 21122112, 21212121, and 21222122. Since 21112111 is not divisible by three, and 21122112 is divisible by three, the answer is 21122112.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.