????????
      +---------
A691A | A2822343
       -A691A
        =====
         80073
        -A691A
         =====
         6368A4
        -67899A
         ======
          690313
         -67899A
          ======
          ??????

base: 12
letters: 0123456789AB
missing: B 5
difficulty: challenging
hints: B << 6, 4, 5