The number nequals​6,34d,217 is divisible by 11. Since its alternating sum of digits is 6minus3plus4minusdplus2minus1plus7equals​(6plus4plus2plus​7)minus​(3plusdplus​1)equals15minus​d, the divisibility by 11 test means that dequals4. Determine the unknown digit​ a, b, or c in the following​ numbers, knowing that each is divisible by 11.