????????
      +---------
21C51 | 11C4A70D
       -8EFAE
        =====
        3843A7
       -3C00FE
        ======
         089970
        -06F7A1
         ======
          307C4D
         -33A591
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: hard
hints: 6 << 5, C << D
missing: B