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
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