????????
      +---------
668DA | 0C918C80
       -668DA
        =====
        4B7D9C
       -411624
        ======
         1D6198
        -1446AB
         ======
          39B500
         -3DDA16
          ======
          ??????

base: 14
letters: 0123456789ABCD
difficulty: challenging
hints: D << 7