????????
      +---------
BD876 | 8DE47897
       -BD876
        =====
        706528
       - 24783
        ======
          98D19
         -4B368
          =====
          B77DE7
         -6CEBCD
          ======
          ??????

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