a wall too high to see over separates 4 men, 1 man on one side, three men on the other. the three men on their side stand single file facing the wall and can only see the man (men) in front of them. on each is placed a hat, not knowing the color on their own head. they are instructed to determine the color of their own hat and are told: 2 hats are white, 2 are black. After 20 minutes, no one has yet shouted out the answer. Which man determines the color of his hat, and why?