Five years ago, there were 500 old books in the library. Two years ago, the librarian bought 300 books. Last year, the librarian bought 100 more books than she had bought the previous year. This year, the librarian donated 200 of the library's old books. How many books are in the library now? Let's think first. Chain of thought:
Last year the librarian bought 300 + 100 = 400 books. Thus, there were 500 + 300 + 400 = 1200 books as of last year. Therefore, there are 1200 - 200 = 1000 books now in the library.
Therefore, the answer is 1000.