This is the problem that a teacher (Kevi Spacey actually) presented the class on the movie 21: You are on a TV show and the announcer asks you to choose 1 from 3 doors. Behind 2 doors there are a goat and behind 1 there is a new car. The guy coose door 1. The announcer go and opens door 3, in wich there is a goat, and asks the guy if he want to change. He said yes and open door 2 and win the car. Now the teacher asks why the guy changed the door? He answer, at the begining he had 33.3% of chance of being right, but when the announcer (that knew where was the car) asked if he would like to change, he thought that changing would increase his chance of winning to 66.6%. My question is if this calculation is right? After 1 door was opened I think that probabilities would change to 50%. What do you guys tell me?