????????
      +---------
B3B98 | A297B17D
       -989B6
        =====
         6CC41
        -B3B98
         =====
         1581E7
        -50B43E
         ======
          1EB60D
         -1B21D1
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: hard