Like the title says, I'm looking for math problems that are easy to understand, but difficult to see the solutions immediately or even after much thought; but when shown the solution, the solution is easy to understand. There's this "poisoned wine challenge" problem which is a pretty great example of this. (PBS infinite series made a video on that.) It could be an old problem, or a new one, doesn't matter.
P.S. I apologize if this question has been asked before.