There are 120 pages in a book, how many leaves should be inserted every 4 pages?
一本书共120页，每隔4页夹进一片树叶一共要夹多少片树叶？
<gadget id="calculator">120 / 4</gadget>
<output>30</output>

<gadget id="calculator">30 - 1</gadget>
<output>29</output>

<result>29</result>