????????
      +---------
263A8 | 820709C5
       -263A8
        =====
         A8D39
        -263A8
         =====
         24AA0C
        -2AE82B
         ======
           42535
          -AC734
           =====
           ?????

base: 15
letters: 0123456789ABCDE
difficulty: hard
missing: 1