[Gabe]

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How would the last player know whether to guess or not?

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Because he's last. I'm assuming that it goes around the table and each player decides in turn to fold or guess. With 4 players, they should have a strategy where they are better than 15-1 favorites to beat the house, if the deck isn't stacked and the deck is smaller than infinity. Each player passes unless one or more of the remaining players has a red card, if none of them do, they "guess" red. If they all pass the last player "guesses" red.