600 books are placed on 4 bookshelves, each shelf has 5 layers, how many books are placed on each layer on average?
600本书放在4个书架上，每个书架有5层，平均每层放多少本书？
<gadget id="calculator">600 / 4 / 5</gadget>
<output>30</output>

<result>30</result>