The library bought 400 story books and placed them on 4 shelves on average. Each shelf has 5 floors. How many books are placed on each floor?
图书室买来400本故事书，平均放在4个书架上，每个书架有5层，平均每层放多少本书？
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">400 / 20</gadget>
<output>20</output>

<result>20</result>