Puzzle for April 20, 2010

Saturday is the 210th anniversary of the founding of the Library of Congress. The first catalog in 1802 listed 964 books and nine maps. 12 years later there were 3000 books when the library was burnt by British soldiers.

Thomas Jefferson "replaced" the library with 6487 books and by 1851 the library contained 55,000 books. Nowadays the library has more than 17 million books, as well as millions of maps, manuscripts, photographs, films, audio and video recordings, prints, and drawings.

What was the average growth rate between the library's founding and today?

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