I am always on the lookout for problems that do a reasonably good job of demonstrating either the left-brain ‘follow the rules’ thinking, or right-brain flash of insight thinking, or both. People who are great problem solvers are good at both. (In Advantage I talk about people who have this capability in spades - supersynthesizers.)
It is also nice to find a problem where the solution is elusive until the moment you see it, then it is obvious.
The problem below is such a problem. The solution to the problem below requires a flash of insight. I will post the flash of insight tomorrow. If you make a comment, please don’t give
away the flash of insight.