Then you do the math. Once you have your answer look at the symbol to the right of the answer.
Press enter and the same symbol is shown. All answers that are multiples of 9 have that symbol next to them no matter if the symbol changes next try or not.
This trick can be done with any number of digits. My program arranges the numbers and symbols more neatly by using MOD and a PRINT "". It also uses number answers from 99 to 198 for a range between 100 and 200.