0551
      +---------
C3697 | 79025C95
       -C3697
        =====
        08724C
       -06BCB4
        ======
         081A69
        -06BCB4
         ======
          0C6B05
         -036B4C
          ======
           39939

base: 13
letters: 0123456789ABC
difficulty: hard