(US $$1998$$ Math kangaroo Problems, Level $$5-6$$, Question \#$$15$$)When from any three-digit number we subtract that number written backwards, the difference will always be a number that is divisible by:． 
Overseas Competition->Knowledge Point->Number Theory Modules->Remainder Problems->Casting Out Nines
No matter how we write the three-digit number, the sum of its three digits is always the same. Therefore, the remainders of the two numbers divided by $$9$$ are also the same. The difference will always be a number that is divisible by $$9$$. 