Read a book, read 10 pages a day, and finish it in 30 days. If you read 15 pages a day, how many days can you finish reading? (Solve with ratio)
读一本书，每天读10页，30天可以读完．如果每天读15页，多少天可以读完？(用比例解)
<gadget id="calculator">10 * 30</gadget>
<output>300</output>

<gadget id="calculator">300 / 15</gadget>
<output>20</output>

<result>20</result>