Here's a simple question, what is the minimum number of people needed in order to find two people with the same birthday? 367 obviously, and that's for a 100% guarantee that two people will have the same birthday. Now let's do some higher level mathematics.
What is the number of people needed in order for a [u][b]50%[/b][/u] chance of getting two people who have the same birthday? Be careful, the answer isn't as easy as you might think, and the equation to get the answer is just as complicated. If too many people can't figure it out, I'll post the correct answer.
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Like Boot said, 2. Either they will or they won't. 50/50