There are four prisoners in a line with a brick wall separating them to three and one. They are wearing alternative coloured black and white hats, but they do not know this. They only see ahead of them - which prisoner can solve the colour of their hat?

Let the prisoners be named: A(black); WALL; B(white), C(black), D(white)

The middle prisoner in the row of three must know the colour of their hat.

D can see A and B but cannot work out the colour of their hat.

Neither A nor B can see the colour of anyone else's hat.

As D does not say anything, they will all know that B and C are wearing different colour hats. C can see the colour of B's hat and will know they have the other coloured hat.