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Monty rule problem
(lemmy.world)
Be sure to follow the rule before you head out.
Rule: You must post before you leave.
ponder on the fact that the host is not just opening one of the doors you didn't pick at random, they are specifically opening one you didn't open and that had the inferior prize. this reveals new information. if you did choose one of the two bad prizes initially (a two in three chance, right?), then the one that you can switch to must have the good prize.