My birthday party

My birthday party

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I have invited my brother Juan and 21 other friends to celebrate my birthday. At a time of the party, Juan, very fond of ingenuity games, proposes the following puzzle.

What is the probability that two of the people attending my party celebrate the birthday the same day?


There are a total of 23 people at the party: myself, my brother Juan and my 21 friends. The probability is over 50% which is paradoxical since there are only 23 people.

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