Xinhua Bookstore has 900 story books, and after 4 days of selling, there are still 640 books left. How many books are sold per day on average?
新华书店有900本故事书，卖了4天后还剩下640本，平均每天卖多少本？
<gadget id="calculator">900 - 640</gadget>
<output>260</output>

<gadget id="calculator">260 / 4</gadget>
<output>65</output>

<result>65</result>