????????
      +---------
579D7 | 500123A9
       -579D7
        =====
         C2EE3
        -9B2D0
         =====
          21AEA
         -2456A
          =====
           27189
          -579D7
           =====
           ?????

base: 15
letters: 0123456789ABCDE
difficulty: hard