This is a famous problem, called Monty Hall problem after a popular
TV show in America.
In the problem, you are on a game show, being asked to choose between
three doors. Behind each door, there is either a car or a goat. You
choose a door. The host, Monty Hall, picks one of the other doors, which
he knows has a goat behind it, and opens it, showing you the goat. (You
know, by the rules of the game, that Monty will always reveal a goat.)
Monty then asks whether you would like to switch your choice of door to
the other remaining door. Assuming you prefer having a car more than
having a goat, do you choose to switch or not to switch?