Let's abstract this question into Game Theory Language:
Assume that there're 100 players in this game, and everyone is rational. Each player has two actions in his decision node respectively(says "black" or "white" according to question). And the initial node represents the 100th player decision node. Because the hats have been determined in advance and players can merely see the color before him or her, the 100th player (the first one who makes decision) doesn't know his or her own color of hat. his expected payoff is 0.5(assume get 1 if guess right and 0 if not), no matter whether he chooses "black" or "white". And if we draw this game tree, we can find this is a 100-stage perfect game where everyone can observe the upper players' actions. But his or her own payoff depends on the luck. This is the situation where there is no collusion and everyone can hear all signals from his or her back, while he or she does't know whether it is true or not(i.e. he or she can merely hear the words from his or her back, and does't know the players behind him or her is alive or dead).
In this case, every kind of payoff() is Nash eq. because there is no incentive for players to deviate from 0.5 to 0.5. In fact according to the question, what we need to do is to maximize the sum of everyone's payoff(denoted by S). So if there is no collusion, we can not increase the sum of everyone's payoff. S = 50!
Now the question is how they can collude and maximize the sum of payoffs. One way is the former player on the game tree hides some information in his word(eg. tells the latter one his or her color). Assume the even number's players tell the player before his color, then S = 75.
Also the 100th player can hide the even or odd information in their words to sacrifice for the other 99 players(i.e."black"means the quantity of black hats is odd among the 99 players and vice versa. S will be 99.5), but according to the condition that players can only say "black" or "white", that means no information can be involved into the words except "black" and "white".(in another word, collusion should have a limit!)
So the idealest answer is S = 75, I think(If there is no other condition!).
Waiting for the right answer!
[此贴子已经被作者于2005-9-15 2:32:17编辑过]