TITLE: Help: rules of a game whose details I don't remember!
QUESTION [31 upvotes]: In a probability course, a game was introduced which a logical approach won't yield a strategy for winning, but a probabilistic one will. My problem is that I don't remember the details (the rules of the game)! I would be thankful if anyone can complete the description of the game. I give the outline of the game, below.
Some person (A) hides a 100 or 200 dollar bill, and asks another one (B) to guess which one is hidden. If B's guess is correct, something happens and if not, something else (this is what I don't remember). The strange point is, B can think of a strategy so that always ends to a positive amount, but now A can deduce that B will use this strategy, and finds a strategy to overcome B. Now B knows A's strategy, and will uses another strategy, and so on. So, before even playing the game for once, there is an infinite chain of strategies which A and B choose successively!
Can you complete the story? I mean, what happens when B's guess correct and incorrect?
Thanks.

REPLY [6 votes]: I know this is a late answer, but I'm pretty sure I know what game OP is thinking of (and none of the other answers have it right).
The way it works is person A chooses to hide either $100$ or $200$ dollars in an envelope, and person B has to guess the amount that person A hid. If person B guesses correctly they win the money in the envelope, but if they guess incorrectly they win nothing.
If person A uses the predictable strategy of putting $100$ dollars in the envelope every time, then person B can win $100$ dollars every time by guessing $100$ correctly.
If person A instead chooses to randomly put either $100$ or $200$, then person B can guess $200$ every time--he'll win half the time, so again will win $100$ dollars per game on average.
But a third, better option for A is to randomly put $100$ in the envelope with probability $2/3$, and $200$ dollars in with probability $1/3$. If person B guesses $100$, he has a $2/3$ chance of being right so his expected winnings are $\$66.67$. If person B guesses $200$, he has a $1/3$ chance of being right so his expected winnings are again $\$66.67$. No matter what B does, this strategy guarantees that he will win only $\$66.67$ on average.
Looking at person B's strategy, he can do something similar. If he guesses $100$ with probability $2/3$ and $200$ with probability $1/3$, then no matter what strategy person A uses he wins an average of $\$66.67$. These strategies for A and B are the Nash Equilibrium for this game, the set of strategies where neither person can improve their expected winnings by changing their strategy.
The "infinite chain of strategies" you mention comes in if either A or B start with a strategy that isn't in the Nash equilibrium. Suppose A decides to put $100$ dollars in the envelope every time. The B's best strategy is of course to guess $100$ every time. But given that, A's best strategy is clearly to put $200$ dollars in the envelope every time, at which point B should change to guessing $200$ every time, and so on. In a Nash equilibrium, though, neither player gains any advantage by modifying their strategy so such an infinite progression doesn't occur.
The interesting thing about this game is that although the game is entirely deterministic, the best strategies involve randomness. This actually turns out to be true in most deterministic games where the goal is to predict what your opponent will do. Another common example is rock-paper-scissors, where the equilibrium strategy is unsurprisingly to choose between rock, paper, and scissors with equal probability.