1) what's the probability that someone else has thesame birthday as you (assuming neither has a birthday in a leapyear).
2) what's the probability that someone in a room of 20people that one them has the same birthday as you (also assumingneither has a birthday in a leap year).
3) what's the probability that someone in a room of 20people that there will be 3 people that have the same birthday(assuming none have a birthday in a leap year).