In this case I give a rough plan of a river with an island and five
bridges. On one side of the river is a monastery, and on the other side
is seen a monk in the foreground. Now, the monk has decided that he will
cross every bridge once, and only once, on his return to the monastery.
This is, of course, quite easy to do, but on the way he thought to
himself, "I wonder how many different routes there are from which I
might have selected." Could you have told him? That is the puzzle. Take
your pencil and trace out a route that will take you once over all the
five bridges. Then trace out a second route, then a third, and see if
you can count all the variations. You will find that the difficulty is
twofold: you have to avoid dropping routes on the one hand and counting
the same routes more than once on the other.
[Illustration]