????????
      +---------
97D4B | 89E54AA2
       -97D4B
        =====
        C43C6A
       -C1E3D0
        ======
         96B36A
        -9A9DD2
         ======
          D4BAD2
         -DC298E
          ======
          ??????

base: 15
letters: 0123456789ABCDE
difficulty: challenging