How does the computer know what symbol you are looking at

The mathematical equasion that the computer is employing is one that makes any number you chose a multiple of 9. If you are to look at the screen when it tells you to look at the symbol that matches your number, you will realize that 9,18,27,36,45,54,63,72,81,90,99, etc will all be the same symbol. No matter what number you start off with, It will always be one of these.