????????
      +---------
56AD8 | 33708E55
       -56AD8
        =====
        656C9E
       -697D4A
        ======
         100135
        -08651E
         ======
          128EE5
         -08651E
          ======
          ??????

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