????????
      +---------
AB113 | 81646A0B
       -AB113
        =====
        48BACA
       -4A5605
        ======
          1C8B0
         -30554
          =====
          48B92B
         -4A5605
          ======
          ??????

base: 13
letters: 0123456789ABC
difficulty: challenging
missing: 7