The Dead Pool date

[PeteyPirate]

OK, this appears to be somewhat relevant. At the bottom of this mess of nerdspeak is the conclusion that the probability of three people in a group of 24 sharing a birthdate is roughly 1 in 70. For a group of 40, we can assume it would be slightly higher. If we just pick 1 in 60 out of the air, for the sake of argument, we would then have to calculate the odds of it being a particular date (1 in 365).

So 1/60 * 1/365 = 1/29,400 or roughly 5/1,000ths of a percent.

http://mathforum.org/library/drmath/view/56650.html