How does this work?

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.