And how the actual math works...
Let's say X and Y are the two numbers that form the two digit number, X being the tens. X being anything from 1 to 9, and Y being anything from 0 to 9. Z is the result number, the one you look up from the table.
So the math goes like this...
(10X+Y)-(X+Y)=Z
10X+Y-X-Y=Z
9X=Z
So as yoou can see, as X is any number between 1 and 9, Z can only be 9, 18. 27, .... ,81 And all those numbers always have the same symbol, which the stupid mindreading ball then shows.