????????
      +---------
570CB | A4195C92
       -570CB
        =====
        3C8C0C
       - 570CB
        ======
         592B59
        -526211
         ======
          C21C42
         -C45A2C
          ======
          ??????

base: 14
letters: 0123456789ABCD
difficulty: medium
missing: D