Arne has 2n + 1 tickets
Source: Flanders Math Olympiad 2022 p3
December 24, 2022
combinatorics
Problem Statement
Arne has tickets. Each card has one number on it. One card has the number on it. The natural numbers occur on exactly two cards each. Prove that Arne can arrange cards in a row so that there are exactly cards between the two cards with the number , for every .