????????
      +---------
3B21D | 342C45C2
       -3B21D
        =====
         5E135
        -657C8
         =====
          1072C
         -90A2A
          =====
           A7AD2
          -067B3
           =====
           ?????

base: 15
letters: 0123456789ABCDE
difficulty: hard