Thank you for teaching me these methods. I was faced with these types of problems in a geometry book (about dogs, strangely), and after struggling for over an hour, I watched one of your videos, went back and easily solved the problem. On this problem, I did it a different way, but it still worked. I used the rather lengthy equation: 33 - [(17-x) + x + (13-x)] + 25 - [(17-x) + x + (16 -x)] + 27 - [(16-x) + x + (13-x)] = 26. I found the 26 (tourists who had only been to one city) by subtracting the 50 who had not been to any cities from the 76 who had been to at most one city. Now that I have watched this video, I know there may be more efficient options.
