I read a 290-page book for 4 days, and read an average of 20 pages a day. How many pages are left?
一本290页的书，看了4天，天平均每天看20页，还剩多少页没看？
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">290 - 80</gadget>
<output>210</output>

<result>210</result>