grosse gurkes got the right idea. It will only work if you actually do what the instructions say (trust me if you can subract it works everytime) - it wont work if you just pick a number. Piranha 13 i dont know what you mean by "every answer i always 0". But think in terms of sections, like all of the 70's or 60's or 50's, etc. Look at 36, or 45, there are more.
The only possible answers are multiples of 9.....9,18,27 ect. So all the multiples of 9 are the same symbal. If you do the addition and subtraction properly, you will come up with a multiple of 9. Each time you choose a number and go through the entire process, the sysbols change however, everytime the multiples of 9 all change to the same sysbol.