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77 was my starting number.
yea? well i can guess what symbol it is going to show :P
Take the number, equal to 10X + Y, and subtract from it the sum of the two digits, equal to X + Y. The full algebraic expression is 10X + Y - (X + Y), which equals 9X. This proves that the number will always be a multiple of 9.
Example: 32: 3 * 10 + 2 - (3 + 2) = 3 * 9 = 27.
I`m sure some of you already knew that, but for those that didn`t, there`s an algebra lesson for you.