A prison warden assembles 6 death sentenced prisoners and asks them to stand in such way that each one can see all the others. He places a hat on each of their heads, either a black or a white one, there is atleast one black and one white hat. Each person has to identify the color of the hat he is wearing, of course by looking at others hats only coz they cant see theirs and he gives them 20 mins to do so. Nobody can communicate with anybody in any way except to announce the color of their hat to the warden. The condition the warden puts accross is that if everybody gets their own hat colors right then he’ll free everybody and even if one gets his wrong, he’ll kill all. Within the alloted time, everyone shouts out the color of the hat they are wearing and everyone is right. how did they decipher that? ( generally if one sees all blacks or whites, then he can be sure that the one he is wearing is the other color. but for this one let us assume we have atleast 2 of a single color to make it a little more complicated.)
Source: http://answers.yahoo.com/question/index?qid%3D20100903061130AAMHALs