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That is pretty cute. Since I am a total math geek...I went ahead and found out the "magic" behind this trick.
When you chose any two digit number, let's say (10x + y), and subtract the sum from it you get this:
(10x+y) - (x + y)
=10x + y - x - y
= 9x
Therefore, your number can only be a multiple of 9. And if you look at the symbols, you'll notice that every multiple of 9 has the same symbol.
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