????????
      +---------
105CE | 7651485C
       -105CE
        =====
        5E2978
       -54A619
        ======
         930195
        -904A60
         ======
          C3864C
         -CDE7AD
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: hard
hints: 6 << B
missing: B