An old chestnut goes like this:

How many people do you need to have together for the probability that two of them have the same birthday to be at least 50%?

Usually these simplifying assumptions are used:

This is also written up in Birthday paradox (Warning: partial spoiler in link) but it's not really a paradox and the calculation is not described over there, only the answer, so I'm not using that node for this puzzle.


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