????????
      +---------
3B898 | 7B5B5909
       -3B898
        =====
        4047A9
       - 3B898
        ======
         B6B340
        -B88B34
         ======
          262219
         -260A31
          ======
          ??????

base: 12
letters: 0123456789AB
difficulty: intermediate