????????
      +---------
8EC27 | C27802ED
       -8EC27
        =====
        87A4A2
       -892D8E
        ======
          A5C6E
         -BA336
          =====
          67B40D
         -69A0BD
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: hard
missing: 1