Normally I don’t care for combinatorial problems, but this problem from Chalkdust Magazine by Matthew Scroggs seemed to bug me enough to try to solve it. It took me a while to see the proper pattern, and then it was rather satisfying.
“You start at A and are allowed to move either to the right or upwards. How many different routes are there to get from A to B?”
See the Chalkdust Grid Problem