While Danny, Edmund and Frank were out trekking in the wilderness, they were ambushed and captured by cannibals. The cannibals gave the men a possible chance to escape.
Danny, Edmund and Frank are lined up in order, so that Danny can see Edmund and Frank, and Edmund can see Frank only, while Frank cannot see anyone. The cannibals take out 5 hats, of which 3 were black and 2 were white, and show these to the men. The hats were then placed on each man's head (one hat per head), without the men knowing what color their own hat is. The men would be allowed to escape if they can guess the color of their hats.
Danny looks at the hats of the people in front of him and says "I do not know".
Edmund looks at the hats of the people in front of him and says "I do not know".
What should Frank say?
Details
None of these people can see the 5-3 = 2 leftover hats.