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Duplicate Birth Month

A release-checked easy problem for training Combinatorics.

Question

How many people are required to guarantee that two of them will have a birthday in the same month?

Practice focus

This Brainteasers problem is tagged Combinatorics. State the random variables and conditioning information explicitly, then check the result against boundary cases before opening hints or a solution.

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Brainteasers · Marshall Wace, Akuna