Hint for to Puzzle #23: Envelopes, Stick or Change
If you don't get this you shouldn't feel bad. I've had arguments with intelligent people that, even after I've explained it, find it so counter intuitive they just sort of refuse to accept the logic.
The thing to think about is what is the probability that the twenty pound note is still on the table after you have made your selection? And does that change when one envelope is taken away? It may me helpful to pretend that the questioner knows the location of the £20, though it doesn't matter. Or imagine the same puzzle with a 1000 envelopes, 998 of them being removed.
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