ohh mystic algebra, yum, lets call the number you pick
10*x + y
now you add the digits and subtract em
10*x+y- (x+y)
y's cancle out and you are left with :-
9x.
so your end result will be 9 times the first digit of the number you picked, pick a number in the teens you will always get 9, pick a number in the 20s and you will always get 18.
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