Xiaoxiao looked at a story book with 170 pages in total, and she had already read 90 pages. She will read the remaining pages within 4 days, how many pages per day on average?
笑笑看一本故事书共170页，她已经看了90页．剩下的页数，她要在4天内看完，平均每天看多少页？
<gadget id="calculator">170 - 90</gadget>
<output>80</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<result>20</result>