????????
      +---------
96284 | 94A9A9C7
       -96284
        =====
         20799
        -96284
         =====
         69270C
        -6CC772
         ======
          7A31B7
         -733920
          ======
          ??????

base: 13
letters: 0123456789ABC
difficulty: challenging
missing: 5