Take any two digit number, doesnt matter, make x the first digit and y the second digit, so your number is XY XY is really X*10 +Y, e.g. 54 is 5*10 +4 now add the two digits, you get (X+Y) sub the sum of the two digits (X+Y) from the original number (10X+Y) and you get: (10X+Y) - (X+Y) = 10X +Y - X -Y = 9X so the answer will always be a multiple of 9 cause X can be 1, 2, 3, 4, 5, 6, 7, 8, 9 |
