Choose your two digit number. Let's call the number AB. A is the number of tens, B the number of units.
So your number is:
10A + B
Add the two digits together:
A + B
Take this away from your original number:
(10A + B) - (A + B)
10A + B - A - B
9A
The number you are left with will always be a multiple of 9, and will always be less than or equal to 9*9, as the highest two digit number you can begin with is 99. If you look, all the numbers that are less than or equal to 81 and divisible by 9 have the same symbol.
EDIT: Visage beat me by like a zillion posts.