If the digits of any number sum to a multiple of nine then it is also divisible by nine. Rearranging them won't change this.
Well a simple way to do it is to remember that, S(.) being the sum of the digits of a number, (this can be easily proved with congruences, writing the number as powers of 10). And since (rearranging the digits won't change their sum !), then
More generally, if the remainder of the sum of digits of a number, divided by 9, is n, then the remainder of the number, divided by 9, is also n. Since rearranging the digits in a number does not change the sum of digits, it also does not change the remainder when divided by n. Let A be the original number, and let n be its remainder when divided by 9: A= 9k+ n for some integer k. If B is any rearrangement of the digits of A, then B= 9j+ n for some integer j. A-B= (9k+ n)- (9j+ n)= 9(k- j).
You can prove that statement about the remainders by looking at the numbers expansion in powers of 10:
The congruence gives the remainder, so...
And the OP seems to have understood the solution, given the private messages he's sent me...