Use a batch of white paper to pack and type exercise books. Each 30-page book can bind 400 books. If each book has 40 pages, how many exercise books can be bound? (Solve with ratio)
用一批白纸装打练习本．每本30页可以装订400本，如果每本40页，可以装订练习本多少本？(用比例解)
<gadget id="calculator">30 * 400</gadget>
<output>12_000</output>

<gadget id="calculator">12_000 / 40</gadget>
<output>300</output>

<result>300</result>