I am afraid I am going to shatter your dreams of extra sensory perception with a little Algebra to prove this is a fake. (A nice fake mind you LOL)
Let the number you choose be denoted by:………………..xy
………………………………………….This actually represents …..10x + y (because the x is a unit of 10)
………………………………………….Add the digits together gives x + y
………………………………………….Subract this from 10x + y and you always get 9x
Now x can only be either 1,2,3,4,5,6,7,8,9,
And therefore the answer can only ever be 9,18,27,36,45,54,63,72,81,
So all they have to do is create ONE symbol each time (which they vary from game to game) to represent ALL of these possibilities.
To prove this, look at the example I did:
Look at the symbols for 9,18,27,36,45,54,63,72,81,
They are all an oval with a circle inside.So he will never get it wrong.