Multiples of 9. If you look at the chart carefully, all multiples of 9 upto 81 have the same symbol (the symbol changes each time you try but all of them will have the same symbol in one particular round). The question has been crafted to ensure that no matter what number you choose, the result is ALWAYS a multiple of 9 and ALWAYS less than or equal to 81. Therefore the game can always know what your answer will be regardless of your calculation. The numbers 90 and 99 have different symbols than the other multiples of 9 just to confuse the user...they do not matter because you cannot get any result higher than 81 (i.e. 99-(9+9)=81 ). Pretty simple really and I think it has been posted here sometime ago.