The trick is that Monty always removes an empty door.
3 doors:
If you pick the prize, Monty removes an empty door. You switch and you LOSE.
If you pick the empty door, Monty removes the other empty door. You switch and you WIN the prize.
If you pick the other empty door, Monty removes the second empty door. You switch and you WIN the prize.
So switching results in a 2/3 chance to win. Not 50/50.
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