How do these kind of things work?

[wvjules]

I know there is a trick to it, but I can't figure it out. Anyone have any ideas?

 

[Miss Jasmine]

It has to do with multiples of 9.

 

[wvrevy]

Pick two different numbers (say, 76 and 43) and do the expirament with both of them, then look at the symbols for both of them (63 and 36, in that case) and you'll see how they "guess" your symbol.

 

[salmoneous]

Short answer: after subtracting the digits, you will always get a multiple of 9. If you'll notice, each multiple of 9 (18, 27, 36, etc) has the same picture - that's the one they show you.

As for why you always get a multiple of 9: Your number is XY (so if you pick 43, X=4; Y=3). Your number is 10X+Y. You then subtract the sum of the digits (X+Y).

(10X+Y) - (X+Y) = 9X