????????
      +---------
9090B | 63B66937
       -9090B
        =====
        B06499
       -B0886A
        ======
          6A563
         -9090B
          =====
          B50A17
         -B50588
          ======
          ??????

base: 12
letters: 0123456789AB
difficulty: medium
hints: 2 << B
missing: 2