the trick lies in the symbols
realize that the symbols for all the numbers which are multiples of
9 are the same?
the mathematical concept is this:
let the number you chose be XY, where X is the first digit and Y is
the second digit, hence the number is equivalent to 10X + Y
the sum of these two would be X + Y
subtracting X + Y from 10X + Y, one gets 9X which is always a
multiple of 9, hence any multiple of 9 would give you your
answer
cheers
