AIC - Pathway to the World

The AIC Challenge
Ben, Ken, Len and Sven are lined up, all facing in the same direction. A high wall separates Sven from the other three. Ben can see Ken and Len.  Ken can see Len.  Len and Sven can not see any of the others. They are all wearing hats.

They are told that there are two white hats and two black hats, but they don't know what colour hat they are wearing.

They are told to shout out the colour of the hat that they are wearing as soon as they know for certain what colour it is. They are not allowed to turn round or move, or to talk to each other, or to take their hats off. Who can shout out first, and why?

Click here for the answer