Leslie and Marvin, both teachers, are adding books to their class libraries. Leslie's classroom started out with a collection of only 20 books, but she plans to purchase an additional 3 books per week. Marvin's library started out with 40 books, and he has enough money in his budget to purchase another 1 book per week. At some point, the two teachers' libraries will contain the same number of books. How many weeks will that take?