First find the total weight of the hardcover books: 70 books * .5 pound/book = <<70*.5=35>>35 pounds
Then find the total weight of the textbooks: 30 books * 2 pounds/book = <<30*2=60>>60 pounds
Then find the total weight of the knick-knacks: 3 knick-knacks * 6 pounds/knick-knack = <<3*6=18>>18 pounds
Then find the total weight of all the items: 35 pounds + 60 pounds + 18 pounds = <<35+60+18=113>>113 pounds
Then subtract the bookcase's weight limit: 113 pounds - 80 pounds = <<113-80=33>>33 pounds
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