Yep, I was playing around with it and saw that that is the trick. Every time the choices of numbers that can possibly be the answer all have the same symbols. The symbol changes every time you try again, but always 0, 9, 18, 27, 36, 45, 54, 63, 72, 81 will always have the same symbol, and that is the one which will come up in the ball when clicked. If you add and subtract correctly, those are the only numbers that can be answers. I did notice he has a couple of the same symbol scattered around amonst other numbers to throw you off so it won't be readily apparent what the trick is.
And yes, I had used numbers with the first digit being "0".