D488
    +-------
DAA | 637509
     -E56
      ===
      7C45
     -7CB4
      ====
        520
       -5E8
        ===
         449
        -5E8
         ===
         73C

base: 15
letters: 0123456789ABCDE
difficulty: hard
hints: 4 << C
missing: 1