Well mathematically there are only 9 possible answers to the question (9,18,27,36,45,54,63,72 and 81) and the symbol next to each of these numbers is the same. The clever bit comes when you play again the symbol next to all of those numbers simply changes ... so it looks like magic. Very clever illusion.
it works on the 9 times table and anything that is plused it self minus itself = 9. ie A+B-(AB) = C i.e pick number 10 1+ 0-(10) = 9 what ever is next the 9 will appear. if you use the 9 time table each increas has the same symbol next to it.