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The reason why it can predict what your symbol is because it is always in the nine times table. If you add two digits together and take it away from your original number it will take you to the nearest multiple of nine. When you add the digits it will give you the distance it is away from the multiples of 9 because it always increases by 1 and when the number increases the distance away increases so the number away will increase so that the answer is always in the 9 times tables.