????????
      +---------
AC517 | 483A627C
       -AC517
        =====
        16C1E2
       -188924
        ======
         B04587
        -B8301C
         ======
          B20A1C
         -BE26A9
          ======
          ??????

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