A story book has 300 pages, and Xiao Ming reads 20 pages every day. After 13 days, how many pages are left unread?
一本故事书有300页，小明每天看20页，经过13天，还剩下多少页没有看？
<gadget id="calculator">20 * 13</gadget>
<output>260</output>

<gadget id="calculator">300 - 260</gadget>
<output>40</output>

<result>40</result>