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
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
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
This town was therefore the cyclist's destination.
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 one.
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.