There are 600 story books in the Friendship Bookstore, and there are 150 books left after 5 days of selling. How many books are sold per day on average?
友谊书店有600本故事书，卖出5天后还剩150本，平均每天卖出多少本？
<gadget id="calculator">600 - 150</gadget>
<output>450</output>

<gadget id="calculator">450 / 5</gadget>
<output>90</output>

<result>90</result>