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Perpendicular diagonals in 20-gon

Source: 2023 Israel TST Test 5 P1

March 23, 2023
combinatoricsTSTregular polygon

Problem Statement

A regular polygon with 2020 vertices is given. Alice colors each vertex in one of two colors. Bob then draws a diagonal connecting two opposite vertices. Now Bob draws perpendicular segments to this diagonal, each segment having vertices of the same color as endpoints. He gets a fish from Alice for each such segment he draws. How many fish can Bob guarantee getting, no matter Alice's goodwill?