# Poisson Formula Question

1. Apr 16, 2005

### babtridge

Hi there, I'm a bit stuck and was hoping somebody could give me a couple of pointers....

A lecturer wages that at least one pair of students in his class have birthdays on the same day. What is the minimum number of students in his class for him to be likely to win the bet?

I have assumed a Poisson formula for Pm = (n^m).(e^-n)/m!
for the probability of m pairs having their birthdays on the same day, when n is the mean number of such pairs.

I know the answer is 23 but I am really struggling to obtain this.
Any pointers would be much appreciated. Cheers

2. Apr 16, 2005

### CRGreathouse

(To greatly simplify the problem, I'll assume February 29th is a myth.)

With 1 person in the room, there is a 100% chance that there would be no shared birthdays.

With 2 people, the chance of having no shared birthdays is 364/365. With 3 people, the chance is 364/365 * 363/365.

Continue this and you should get the answer. Remember, you're looking for the chance of having no shared birthdays to fall under 50%.

3. Apr 16, 2005

### babtridge

OK, thats nice and clear now. Thanks for your reply CRGreathouse!

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook