well, doing the math problem they gave you, you can only end up with multiples of 9. see?
10..... 1+0=1..... 10-1=9
11..... 1+1=2..... 11-2=9
12..... 1+1=3..... 12-3=9
(so if you pick any number between 10 and 19, you'll always end up with 9
then like.. if you pick anything in the 20's, you'll always end up with 18
20..... 2+0=2..... 20-2=18
21..... 2+1=3..... 21-3=18
22..... 2+2=4..... 22-4=18
so anything in the 20's you'll always end up with 18
and so on.. anything in the 30's you'll end up with 27, anything in the 40's you'll always end up with 36.
So they always make sure all the multiples of 9 are the same symbol. check it out,
9
18
27
36
45
54
63
72
81
90
99
will always be the same symbol. they switch out which symbol it is each time, but those numbers will always be the same one.