I wrote down the worked example and now I get it:
1) You have 100 doors to choose from - you choose number 69 - your chance of being correct is 1/100
2) The quizmaster now proceeds to open doors 1-68 and doors 70-99. He leaves door 69 (your choice) and door 100 (the quizmaster's "choice") closed
3) The chance of you having picked the correct door with your first choice is 1 in 100 - so 99 times out of 100 you would have been wrong with that original choice. Therefore 99 time out of 100 the quizmaster's "choice" (i.e. the other door he chooses to leave closed) will have the car behind it. So you should definitely change
Phew my brain hurts now - time for lunch.
1) You have 100 doors to choose from - you choose number 69 - your chance of being correct is 1/100
2) The quizmaster now proceeds to open doors 1-68 and doors 70-99. He leaves door 69 (your choice) and door 100 (the quizmaster's "choice") closed
3) The chance of you having picked the correct door with your first choice is 1 in 100 - so 99 times out of 100 you would have been wrong with that original choice. Therefore 99 time out of 100 the quizmaster's "choice" (i.e. the other door he chooses to leave closed) will have the car behind it. So you should definitely change

Phew my brain hurts now - time for lunch.


