It doesn't work all the time?
When?
Looks like it follows:
(10) + X -X-(1) = 9
(20) + X -X-(2) = 18
(30) + X -X-(3) = 27
etc.
All the answers are mulitples of 9 up to 81
Numbers above 81 are not possible and just throw you off.
The symbols are shuffled and re-applied to the mulitples of 9.
Am I missing something, or just stating the obvious. :-)