Birthday paradox

Created 10th September, 2009 07:48 (UTC), last edited 10th September, 2009 08:14 (UTC)
Birthday paradox

Because it seems to be birthday season around me at the moment, another birthday puzzle.

I'm sure pretty much everybody already knows about the Birthday Paradox. The birthday paradox is that you need surprisingly few friends to have two that share a birthday.

How many friends do you need for a 50% chance that two share the same birthday? How many for a 90% chance?

Let's say I know 150 people through the South East Asian BarCamps. If I only know that 2 of them share a birthday what does that tell me about how many of their birthdays I know.


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