Your answer is always a multiple of 9 (eg its on the 9x table) this occurs because if your original number has two digits, a followed by b then it's value is 10a +b. Then when you subtract a+ b from this you will always get 9a left as your answer - eg a multiple of 9
If you study the symbol screen that comes up you will see that each time all the multiples of 9 have the same symbol - so the computer "knows" your symbol because it "knows" your answer is a multiple of 9.
not magic - algebra!
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