Magic Trick: how?

This link was sent to me recently and I fell for it. How does it work?

Give it a moment to load …


There’s only a limited number of answers - any number in the twenties will return ‘18’, anything in the thirties ‘27’ etc. You’ll notice there’s also only a limited number of symbols and - surprise! - all the numbers that are a valid solution have the same symbol (although it changes each time you try).

Drat, on preview, I see Ponster got there first.

Thanks, Bras and Ponster. Now I can sleep easy.

The sneaky bit is that the image with the symbols changes each time that you play !