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I read through a rather extensive explanation... it still doesn't make sense to me.
Try the demo on the link I posted. A dozen or so trials are enough to illustrate the difference.
The key point is not the reduction in the number of doors, but the alteration in conditional probabilities based on new evidence.
Assuming the initial pick is A, the three equally likely options are:
1. Prize is in A, shown non-prize in B or C
2. Prize is in B, shown non-prize in C
3. Prize is in C, shown non-prize in B
In 2/3 of cases (cases 2 and 3), the prize can therefore be won by switching.
Yes, because when you pick option A, you have a 1/3 chance of being correct, but if you switch to B, that is one option between two, giving you a 1/2 chance of being right. It might seem counterintuitive, but it does actually increase your chances of picking right.
Actually, if you switch, you have a 2/3 chance of being right. Try the demo.
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