????????
      +---------
B0660 | 02222739
       -B0660
        =====
        12AA67
       - B0660
        ======
         909083
        -9B87A9
         ======
          1A6589
         -18A5BA
          ======
          ??????

base: 12
letters: 0123456789AB
missing: 4
difficulty: hard