Lizzy: 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?.
Me: Hmmm, let me think. I think this is the detailed solution:
When 90% full, Karson's library holds 90 / 100 * 400 = 360 books. If he has 120 books in the library now, he needs to buy 360 - 120 = 240 books to make his library 90% full. Final answer: 240.