????????
      +---------
B998B | 697B2117
       -B998B
        =====
        797501
       -7895A3
        ======
         B79B11
        -79951B
         ======
          798377
         -7895A3
          ======
          ??????

base: 12
letters: 0123456789AB
difficulty: hard
missing: 4