????????
      +---------
A7033 | 976727B6
       -A7033
        =====
         C05C7
        -757C1
         =====
         A634AB
        -A999CB
         ======
          A61946
         -A999CB
          ======
          ??????

base: 13
letters: 0123456789ABC
difficulty: hard
missing: 8