[Illustration]
The above diagram is the plan of an underground railway. The fare is
uniform for any distance, so long as you do not go twice along any
portion of the line during the same journey. Now a certain passenger,
with plenty of time on his hands, goes daily from A to F. How many
different routes are there from which he may select? For example, he can
take the short direct route, A, B, C, D, E, F, in a straight line; or he
can go one of the long routes, such as A, B, D, C, B, C, E, D, E, F. It
will be noted that he has optional lines between certain stations, and
his selections of these lead to variations of the complete route. Many
readers will find it a very perplexing little problem, though its
conditions are so simple.