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
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
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?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
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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