The birthday room
People walk into a room one at a time. A year has 365 days.
How many people must be in the room before it is more likely than not that two share a birthday?
Where these numbers come from
With 23 people the chance that at least two share a birthday is about 50.7%; there are 253 pairs among them.
Birthday problem — Encyclopaedia Britannica(checked 2026-09-26)