????????
      +---------
C1880 | 483B3C51
       -C1880
        =====
        B41C7C
       -BC9359
        ======
          87415
         -0599B
          =====
          B41A61
         -BC9359
          ======
          ??????

base: 13
letters: 0123456789ABC
difficulty: challenging
missing: 2