????????
      +---------
41ABC | 79A2702C
       -41ABC
        =====
        5B7360
       -53B937
        ======
         684262
        -091C9B
         ======
          51913C
         -53B937
          ======
          ??????

base: 13
letters: 0123456789ABC
difficulty: challenging
hints: 3 << 4