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INMO 2021 Problem 3

    Betal marks 2021 points on the plane such that no three are collinear, and draws all possible line segments joining these. He then chooses any 1011 of these line segments, and marks their midpoints. Finally, he chooses a line segment whose midpoint is not marked yet, and challenges Vikram to construct its midpoint using only a straightedge. Can Vikram always complete this challenge?

    Note: A straightedge is an infinitely long ruler without markings, which can only be used to draw the line joining any two given distinct points.

    Dataset text NuminaMath olympiads. Quoted with source.

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