????????
      +---------
B5D60 | 6A48A49B
       -40FFD
        =====
        B5DFE4
       -B65D29
        ======
         B077E9
        -B65D29
         ======
          BD5E2B
         -BA3441
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: hard
missing: C