Quote:
but I still don't understand why it's not 50/50.

Because your choice made Monty's choice non-random.

If you hadn't chosen first, then yes it would be 50/50. But you did choose first, so your choice forces Monty's choice 2 out of 3 times.

Reread my explanation and Ninti's explanation very carefully and think about each line. The "aha" is in there, you just have to think about it.
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Tony Fabris