The library buys back 240 books, put them on 3 bookshelves on average, and put 4 layers on each bookshelf, how many books are placed on each shelf on average?
图书馆买回240本图书，平均放在3个书架，每个书架上放4层，平均每个书架每层放多少本？
<gadget id="calculator">240 / 3 / 4</gadget>
<output>20</output>

<result>20</result>