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?
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.
#### 240