????????
      +---------
92A21 | 47C13302
       -92A21
        =====
        710A13
       -77AC06
        ======
         6A93A0
        -607AB7
         ======
          7B6292
         -6380A2
          ======
          ??????

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