>>12198309pick one door out of 3
then they reveal which door was right
you'll get it 1/3 of the time
lets go back
pick one door out of 3
then they reveal a door that isnt yours and is wrong, then immediately reveal which door is right
you got no other choice inbetween so you're only right 1/3 of the time again since its the same as above
lets go back one final time
pick one door out of 3
then they reveal a door that isnt yours and is wrong, then they give you a choice to switch
if you dont switch then this is just the situation as above, so you have 1/3 chance of winning
if you do switch, then you must have 1 - 1/3 chance of winning, since the 2 probabilities must add to 1
