1. The problem statement, all variables and given/known data Given a positive integer n written in decimal form, the alternating sum of the digits of n is obtained by starting with the right-most digit, subtracting the digit immediately to its left, adding the next digit to the left, subtracting the next digit and so forth. For example, the alternating sum of the digits of 180,928 is 8-2+9-0+8-1= 2. Justify the fact that for any nonnegative integers n, if the alternating sum of the digits of n is divisible by 11, then n is divisible by 11. 2. Relevant equations There is no relevant equations. The topic is direct proof and counterexample 3. The attempt at a solution By exhaustion, it works, but I did not find any algebraic way to prove it.