Ning Ning reads a 356-page storybook, and reads an average of 21 pages a day. After 13 days of reading, how many pages are left unread?
宁宁看一本356页的故事书，平均每天看21页，看了13天后，还剩多少页没有看？
<gadget id="calculator">21 * 13</gadget>
<output>273</output>

<gadget id="calculator">356 - 273</gadget>
<output>83</output>

<result>83</result>