Smiling and reading a 170-page story book, she has already read 90 pages. The rest should be read within 4 days. How many pages do you read on average per day?
笑笑看一本170页的故事书，她已经看了90页．剩下的要在4天内看完，剩下的平均每天看多少页？
<gadget id="calculator">170 - 90</gadget>
<output>80</output>

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

<result>20</result>