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    OlympiadMediumCombinatorics5–6

    Pigeonhole Principle

    Princípio das Gavetas

    In a group of 13 people, prove that at least two of them were born in the same month.

    There are 12 months and 13 people. Apply the Pigeonhole Principle.

    Solution

    Step 1 of 4

    1. 1.By the Pigeonhole Principle: