I feels smart in my pink wig and moustache!

There are only a few numbers (like probably 10) that the expression can work out to. For example, if you pick 82, it works out to 72 (82-(8+2)).

If you pick 83, it works out to 83-(8+3) = 72.

If you pick 84, it works out to... you got it... 72.

Another possible number is 9. For example all the numbers 10-19 work out to 9.

So there are only a few numbers that it can work out to, right? So how's it guess the symbol? If you look at the chart you'll see that the possible numbers have the same symbol every time. It's just that the symbol that is assigned to that group of numbers changes.

So if you launch the puzzle twice, and look at the 9 and the 72 each time, you'll see what I'm talkin' about.