????????
      +---------
DFCA5 | BA2AA4A5
       -DFCA5
        =====
        858774
       -898468
        ======
         96378A
        -8109AF
         ======
          88A5B5
         -0F95D5
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: medium
missing: E