There are 100 prisoners on an island. Starring from Day 1, a guard will play the following game with them. On the n th day, a random prisoner will be brought into a room, which contains a switch that is either on or off. After he left the room, the guard will then ask the prisoner if everybody has been to the room. He can answer either "Yes" or "I dun know". If he answered "yes" and he is correct, everyone is free' otherwise everyone dies. If he answered "I dun know", the game continues: a prisoner will be choosen at random on (n+1)-th day and so on. Now these 100 prisoners are free to meet and discuss a strategy on the night of 0th day. Afterwards they will not be able to see anyone else at all. No other communcation is possible except the binary switch in the room. Is there a sure-fire strategy to free all of them?
