The Birthdaybirthday problem or birthday paradox is a famous problem in combinatorics and probability theory. The problem is often stated as follows:
Suppose you've gotthere is a classroom full of people. What'sWhat is the probability that that there are two people with the same birthday? How many people there needs does there need to be in the classroom, so that the probability that two people share share the same birthday is 50%?
For a popular introduction checksee the article Probability and the Birthday Paradox article in Scientific American.