Spoiler Warning
Do the math on every number.
For every series the answer is the same. 11-2=9, 12-3=9, 21-3=18, 22-4=18,
So 9, 18, 27, 36, 45... etc. all have the same symbol.
You have to be looking at one of them. Of course every time it comes up it's a different symbol.