Alright you all figure this one out...

[Baby Lee]

Originally Posted by Hydrae

All number divisible by 9 have the same symbol (so does 0).

And every time you follow those instructions, you end up with either 0 or a number divisible by 9, because every number divisible by 9 is x away from the next highest multiple of 10, with x being the number on the 10 spot, and the number in the one spot is y away from the next lowest multiple of 10.

example - 25.
18 is 2 away from 20 and 25 is 5 way from 20, so adding 2 and 5 equals 7, 25-7 is 18.
or
- 87
72 is 8 away from 80 and 87 is 7 away from 80, so adding 8 and 7 equals 15, 87-15 is 72.