There are $n$ people in room each being put on hat from amongest at least $n$ white hats and $n-1$ black hats. They stand in a queue, so that everyone can see the colour of the hat of the person standing in front of him. 

Starting from the back we ask the person in turn, "Do you know the what is the colour of your hat?" If the first $n-1$ person say **no**, prove that front person will say "Yes colour of my hat is white."

![original question](https://i.stack.imgur.com/3bhNH.png)