Use a batch of paper to bind exercise books, if each book has 90 pages, you can bind 40 books; if you want to bind 50 books, how many pages per book? (Solve with ratio)
用一批纸装订练习本，若每本装订90页，可装订40本；若要装订50本，每本多少页？(用比例解)
<gadget id="calculator">90 * 40</gadget>
<output>3_600</output>

<gadget id="calculator">3_600 / 50</gadget>
<output>72</output>

<result>72</result>