When 90% full, Karson's library holds 90/100*400 = <<90/100*400=360>>360 books.

If he has 120 books in the library now, he needs to buy 360-120 = <<360-120=240>>240 books to make his library 90% full.

# Answer

240
Question:
The capacity of Karson's home library is 400 books. If he currently has 120 books, how many more books does he have to buy to make his library 90% full?
Answer:
Let's think step by step.

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240
The capacity of Karson's home library is 400 books. If he currently has 120 books, how many more books does he have to buy to make his library 90% full?
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