Try this...

Nice math trick, anyway

Yes, but the specific numbers don't. What you'll find is that the symbols that come up are used on the ONLY numbers that can possibly be correct. The symbols change each time, but the possible correct answers are the same every time.

It's a math trick. The answer is always going to be a multiple of 9. So all they have to do is use the same symbol for every multiple of 9 and it will always be the correct symbol.