I’ve ignored this riddle for a long time because I thought the solution is simple and that I know it. Recently I found out there is a different and much better solution.
Prisoners
Prison guards are bored so during dinner they announce the prisoners that they will play a game:
In the morning they will stand in a line so that every prisoner will see all standing in front of him but no one behind him. Guards will randomly paint red or white stripe on their back.
They will ask prisoners one by one from the last to the first. They will be allowed to say just one word “red” or “white”. If the prisoner says the color he has on his back he may go home in opposite case he will be executed without delay.
The prisoners are in one cell during the night and they can discuss the tactics.
Which is the best? How many will survive?
Hint
Much more than half can survive - believe me.
I’ll tell you that the solution coheres with parity.
Answer
Prisoners agree that one color means odd number and the other means even number. The last prisoner in row counts (lets say) white stripes and says the color according to their agreement (one color even number of whites other color odd number). He has 50% chance to survive.
The second last prisoner will also count white stripes and if he counts the same parity (odd or even) he knows he has white. If he counts different parity he knows he has red stripe.
Other prisoners will do the same only they have to pay attention to how many times they’ve heard “white”...
So all but one will be released for sure and even the one has a good chance!
(Off course if they don’t screw it up)
