????????
      +---------
DDD57 | E600555E
       -BBBCC
        =====
        4A4C95
       -444425
        ======
         D19615
        -D118E8
         ======
           9E0DE
          -BBBCC
           =====
           ?????

base: 15
letters: 0123456789ABCDE
difficulty: hard
missing: 3