????????
      +---------
C7631 | 6F190A7D
       -C7631
        =====
        76964A
       -71AE02
        ======
         8433A7
        -81F083
         ======
          9F6D8D
         -9CC2B3
          ======
          ??????

base: 16
letters: 0123456789ABCDEF
difficulty: challenging
hints: F is even
missing: 5