Marilyn's Numbrix Puzzle is my second wake-up activity. This was the toughest one I've encountered. Usually I can do them in about 3 minutes regardless of difficulty level. This one took me two tries of over five minutes each. I might have been able to fix the first one, but it was easier to just start over. I've sometimes wondered how difficult you could make them, and what it would take to make a solution unique, like what is the minimum number of squares you could preset that would require a unique solution. But I've got other fish to fry so I'm not going to worry about it.