Xiao Ming reads a story book for 4 days, and reads 5 pages every day, and there are 20 pages left. How many pages does this book have?
小明看一本故事书，看了4天，每天都看5页，还剩20页，这本书有多少页？
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<result>40</result>