[MCS]

Legend42 gave a good answer. [img]/images/graemlins/smile.gif[/img]

Let T be the tens digit and U be the units digit of the number you pick. Then TU is equal to (10*T + 1*U). Now your calculation is:

(10T + U) - (T + U) = 9T

So the answer is always a multiple of 9, and is at most 9*9=81. If you look at the chart, you'll see that all the multiples of 9 up to 81 have the same symbol, which is what the crystal ball displays.