????????
      +---------
07407 | 78798099
       -07407
        =====
        53BB30
       -594594
        ======
         8B0809
        -804804
         ======
          504499
         -594594
          ======
          ??????

base: 12
letters: 0123456789AB
missing: 2 1 A 6
difficulty: challenging
hints: 2 < 6, A < 5