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Originally Posted by kiwi_2005
Ahh... not my own words im to  -
Was told this: The symbols rotate each time you try it, so the symbol will be different each time. It seems that no matter the number you end up with is dividable by 9.
Count from zero, every ninth number has the same symbol. |
Indeed. Easy to prove. You take some random two-digit number. We can represent that as:
a * 10 + b
where a and b are natural numbers from 0 to 9.
Then, the second number is the sum of those digits:
a + b
Finally, you subtract the second from the first:
(a*10 + b) - (a + b) = a*9
Given that a is a natural number smaller than 10, it's pretty obvious that this has to be a multiple of 9, at most 81.