A storybook has 540 pages, and Xiaohua reads about 30 pages a day. After reading it for 4 days, how many pages are left?
一本故事书有540页，小华每天约看30页，看了4天后，还剩多少页没有看？
<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<gadget id="calculator">540 - 120</gadget>
<output>420</output>

<result>420</result>