????????
      +---------
355E4 | 3E252619
       -355E4
        =====
         97306
        -355E4
         =====
          1B0D1
         -355E4
          =====
          89CC39
         -83B82C
          ======
          ??????

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