In any gathering of people, how many must there be to be 50% sure that at least two people will share the same birthday?
Answer: At least 27 people.
How many people at that gathering must there be to be virtually certain that two of them will share the same birthday?
Answer: At least 57 people.
As people enter a room one at a time, which one is most likely to be the first to have the same birthday as someone already in the room?
Answer: The 20th person to arrive.
What is the average number of people (selected at random) required to find two with the same birthday?
Answer: On average, 25 people are required.
[Example: There have been 27 Prime Ministers of Australia. Paul Keating, the 24th Prime Minister, and Edmund Barton, the first Prime Minister, share the same birthday, 18 January.]
Source: All this and more can be found at: http://en.wikipedia.org/wiki/Birthday_problem