158 Combinatorial & Topological Problems
in a different department. For the
purpose of pedestrian exercise he
crossed every one of the bridges once,
and once only.
The puzzle is to show in which two
departments their houses are situated.
It is exceedingly easy if you give it a
few moments' thought. You must not,
of course, go outside the borders of
the diagram.420. FOOTPRINTS IN THE SNOW
Four schoolboys, living respectively in the houses A, B, C, and D, attended
different schools. After a snowstorm one morning their footprints were ex-
amined, and it was found that no boy had ever crossed the track of another
boy, or gone outside the square boundary. Take your pencil and continue their
tracks, so that the boy A goes to the school A, the boy B to the school B, and
so on, without any line crossing another line.