????????
      +---------
13B40 | C3E6D54C
       -13B40
        =====
        2A28E5
       - 8505E
        ======
         238814
        -2C607B
         ======
          2B840C
         -267B14
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: medium
missing: 9