When the children in Sapporo arrived for class yesterday, the teacher saw that 13 were wearing hats, 6 were wearing watches and 10 were wearing trousers.

On closer inspection, he could see that all of the children wearing watches were also wearing

hats, and that half of the children with a watch also wore trousers. There were 4 children

wearing hats who also had trousers on but no watch.