There are 4 bookshelves in the library, each shelf has 4 floors, and each floor contains 40 books. How many books are there in the library?
图书馆里有4个书架，每个书架有4层，每层有40本书，图书馆里一共有多少本书？
<gadget id="calculator">40 * 4 * 4</gadget>
<output>640</output>

<result>640</result>