Alright, but it's going to ruin the "magic". . .
As an example, let's take 99.
9 + 9 = 18
99 - 18 = 81.
Now let's take the next number down, 98.
9 + 8 = 17
98 - 17 = 81
Since you're always subracting one from both the original number and the sum of the two digits, they cancel each other out. The only possible answers are going to be 81, 72, 63, 54, 45, 36, 27, 18 and 9. It looks like the symbol list randomizes itself to keep you from catching on as well.
Now
this will drive you nuts.