What is the probability of in a room with 15 people, there will be at least one shared birthday?
from Google:
In a room of just 23 people there's a 50-50 chance of two people having the same birthday. In a room of 75 there's a 99.9% chance of two people matching. Put down the calculator and pitchfork, I don't speak heresy. The birthday paradox is strange, counter-intuitive, and completely true.