Sharing birthdays
What would you do if you were invited to two birthday parties on the
same day?
Well, you would probably go to the party of the person who had asked you
first. That would be the fair thing to do. But, you’re probably
thinking, there’s not much chance of being invited to two birthday
parties on the same day.
Let’s say you have thirty friends. With 365 days in a year, the chance
that two friends will have the same birthday is pretty slim, right?
Wrong! The chances are actually better that two people out of any thirty
will have the same birthday than that all thirty people will have
different birthdays.
There’s a simple reason for this. With billions
of people in the world, and only 365 days on which to have birthdays,
millions of people share the same birthday. So, in any fairly large
group of people, chances are that two or more people will have the same
birthday. Among thirty people, the chances are two-to-one that two of
them will have the same birthday.
To see how this works, make a “survey” of your family and friends. Find
out the birthdays of ten people. Chances are that you won’t find anyone
with the same birthday. Now, find out the birthdays of ten more people.
You may find two people who have the same birthday among the twenty
people. Finally, find out the birthdays of ten more people. And among
all thirty you’ll probably find two or more people with the same
birthday. Try it and see.
