A classic book has 540 pages in total, and 180 pages have been read. The rest read an average of 18 pages a day, how many days?
一本经典读物共有540页，已经看了180页．余下的平均每天看18页，几天看完？
<gadget id="calculator">540 - 180</gadget>
<output>360</output>

<gadget id="calculator">360 / 18</gadget>
<output>20</output>

<result>20</result>