Easily understood concept...
So the "trick" behind it isn't even really a trick. Besides that, with the way its setup, you leave too much room for error on the user's part. Like that guy who said "19" doesn't work... it DOES work, its just that he missed one of the boxes containing 19. So, as soon as he made a mistake, he blamed it on your "game" and immediately thought it was poorly coded/designed. I'll admit that it is poorly drawn/designed, but I am sure all your little numbers work when selected. Really though, it doesn't matter... thius is worht half a look for 15 seconds and not much more.