User talk:Martin Hogbin/Monty


Car hidden behind Door 1Car hidden behind Door 2Car hidden behind Door 3
Player initially picks Door 1
Player has picked Door 1 and the car is behind itPlayer has picked Door 1 and the car is behind Door 2Player has picked Door 1 and the car is behind Door 3
Host opens either goat doorHost must open Door 3Host must open Door 2
Host opens Door 2 half the time if the player picks Door 1 and the car is behind itHost opens Door 3 half the time if the player picks Door 1 and the car is behind itHost must open Door 3 if the player picks Door 1 and the car is behind Door 2Host must open Door 2 if the player picks Door 1 and the car is behind Door 3
Switching loses with probability 1/6Switching loses with probability 1/6Switching wins with probability 1/3Switching wins with probability 1/3
Switching loses with probability 1/3Switching wins with probability 2/3

Solution

Say you initially pick either of the goats, the host must open the other door with a goat behind it in order to reveal a goat. So you have one door with a goat behind it and the host has opened the other door which also has a goat behind it. The remaining door therefore must have the car behind it. So if you swap, you get a car.

If you initially pick the car, it makes no difference which door the host opens, any other door has a goat behind it. so if you swap, you get a goat.


Player has picked a goatPlayer has picked a car
Player has picked Door 1 and the car is behind Door 2
The host opens the other goat doorThe host opens the either goat door
Players who chose a goat and switch must get a carPlayers who chose a car and switch must get a goat