How does this work?
 

Re: How does this work?
 

All the possible answers are the same symbol. There's only 9 possible answers to

10j + k - j - k ( = 9j)

where j is from 1 to 9.

The first time I tried it, I got 'x,' which was correct. Every time after that, I got 'm.' This was also correct, but I tried to go back and figure out what I chose to get 'x' the first time, and couldn't remember. I guess I'll have to do the math...

The symbols correspond to different numbers each time. Pick a number (say, 36), then look at the symbol (and remember it). Click the ball. Now, hit the back button on your browser and look at the symbol next to 36. It's probably different. And if you click the ball, that symbol will be the one that is shown.

"So I see," said the blind man to his deaf daughter.

They change the symbols. But all the multiples of 9 will always have the same symbol.

Right. I wasn't real clear on that in my explanation. All the possible answers will have the same symbol. For example, do it with 3 numbers - say, 80, 45 and 93. 80 - 8 - 0 = 72. 45 - 4 - 5 = 36. 93 - 9 - 3 = 81. 72, 36 and 81 will all have the same symbol, always (because they are multiples of 9). That symbol might change (because it is arbitrary), but the important part (that all multiples of 9 have the same symbol) will not.