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Combinatorics

Source: Pan African Mathematics Olympiad P3

August 15, 2024
PAMO 2024

Problem Statement

Given an integer n1 n \geq 1 , Jo-Ané alternately writes crosses (X \mathcal{X} ) and circles (O \mathcal{O}) in the cells of a square grid with 2n+1 2n + 1 rows and 2n+1 2n + 1 columns: she first writes a cross in a cell, then a circle in a second cell, then a cross in a third cell, and so on. When the table is completely filled, her score is calculated as the sum X+O \mathcal{X}+ \mathcal{O} , where X \mathcal{X} is the number of rows containing more crosses than circles and O \mathcal{O} is the number of columns containing more circles than crosses.
Determine, in terms of n n , the highest possible score that Jo-Ané can obtain..