The library buys back a batch of new books, 60 books are placed on each floor, and each bookshelf has 4 floors, and a total of 15 such bookshelves are filled. How many new books did the library buy back?
图书馆购回一批新书，每层摆60本书，每个书架有4层，共摆满了15个这样的书架．图书馆共购回了多少本新书？
<gadget id="calculator">4 * 15</gadget>
<output>60</output>

<gadget id="calculator">60 * 60</gadget>
<output>3_600</output>

<result>3_600</result>