The Birthday Paradox

Suppose you are sitting in a classroom with 40 people (including yourself). Now suppose a gunman held a .45 caliber to your head and asked you this question: What are the chances two people in this room have their birthdays on the same day? If you answered 11% which would probably be the safest guess, you would get your head blown off. The actual answer is closer to 90%. In a room of 30 students that probability is still 71%. This chart breaks down the group size to get an approximate probability.



This mathmatical probability problem was proposed in 1939 by Richard von Mises. The implications of this problem are far reaching. You and your friends can systematically take hundreds of dollars off of pre-schoolers to college students by posing this question and then taking the appropriate position. Math is on your side.