????????
      +---------
71728 | B76A0871
       -BB3908
        ======
         991887
        - 71728
         ======
          A51071
         -A20928
          ======
          ??????

base: 12
letters: 0123456789AB
missing: 4
difficulty: challenging
hints: 1 << 6, A << B, 7 << 2