????????
      +---------
83167 | 971E278E
       -83167
        =====
        4E1B07
       -4EC395
        ======
          21E38
         -B5253
          =====
          8DDEEE
         -8D7616
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: hard
missing: A