????????
      +---------
75D55 | 185C6327
       -75D55
        =====
        F1FF73
       -F78CA2
        ======
         079722
        -00A1AE
         ======
          09E2E7
         -014E59
          ======
          ??????

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