It's set up so all possible answers have the same symbol, and it looks like all tens give the same answer, ie. 99 is 9+9=18, 99-18=81, and 96 is 9+6=15, 96-15=81 too, so the answer for all the 90s will be 81. Plant the symbol on 81. For the 60s it would be 54. All multiples of 9 as pointed out earlier.
The symbols chart changes for iteration, but the chart plants the symbols for that iteration on all possible answers.