Dirty F

Still then. Because there is still a 2/3 chance it's behind one of the 2 other doors. No matter what.

Edit: i now understand you. You mean he already opened the door. That doesn't make sense. I mean that's a completely different situation.
Then there simply are 2 doors. Not 3.

Slappin Fish

Well no because he ruled out a door. 1 pick left. 2 doors left. 1/2

Edit : saw your edit yeah that's what I mean. Your title is correct but this example for some reason often used is actually a slightly different question

Dirty F

Then you know the other door that you didn't pick originally has a 2 in 3 chance of having a car behind it.

woj

Slappin Fish is right, the solution changes if by "door #1" and "door #3" you mean a specific door, not just a label to illustrate the problem...

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Jel

no he isn't - watch the video link I posted a couple posts back.

Dirty F

No, it does not.

mikesouth

thats the point if he always knows and always pics one of the two doors that are NOT the right one the odds if you change are 2/3

if he doesnt know and randomly pics a door (and 33% of the time it has the car) then the odds are also 66%

there are variations

lets say he doesnt always off the choice, since he knows where the car is he can manipulate things....say you guess correctly and that is the only time he offers the choice or he only offers it if you guess wrong.

but those strategies would be easy to figure out

NOW it can be shown that if he always opens a door when the contestant is right and half the time when he's wrong--a perfectly rational approach--over the long haul the odds of the prize being behind Door #1 versus Door #2 are 50-50.

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Mike South

It's No wonder I took up drugs and alcohol, it's the only way I could dumb myself down enough to cope with the morons in this biz.

Jel

http://www.bbc.co.uk/learningzone/cl...ned/11261.html

If you pick a door, before there are only 2 doors left, there is never a 50/50 chance at any point. Forget the fact there are only 2 doors LEFT, the fact is you are picking when there are more than 2 doors, so the odds are never 50/50, regardless if you whittle unopened doors from 3 (or a hundred, to grasp it) doors to 2 doors.

Jel

I posted the clip again because it's obvious no-one watched it.