Put a 1 on all the towns in the top row and in the first column.
Then the number
of routes to any town will be the sum of the routes to the town immediately
above and to the town immediately to the left.
Thus the routes in the second row
will be 1, 2, 3, 4, 5, 6, etc., in the third row, 1, 3, 6, 10, 15, 21, etc.; and
so on with the other rows.
It will then be seen that the only town to which
there are exactly 1,365 different routes is the twelfth town in the fifth
row : the one immediately over the letter E.
This town was therefore the cyclist's
The general formula for the number of routes from one corner to the corner
diagonally opposite on any such rectangular reticulated arrangement, under the
conditions as to direction, is
(m + n)!/m!n!, where m is the
number of towns on one side, less one, and n the number on the other side, less
Our solution involves the case where there are 12 towns by 5. Therefore
m = 11 and n = 4.
Then the formula gives us the answer 1,365.