????????
      +---------
20855 | 511618A0
       -20855
        =====
        2647B8
       - A400B
        ======
         26BABA
        - A400B
         ======
           30A30
          -3BB77
           =====
           ?????

base: 12
letters: 0123456789AB
missing: 9
difficulty: easy