There are 600 exercise books in the stationery store. After selling some, there are 4 packs left, each pack of 25 books. How many copies are sold?
文具店有600本练习本，卖出一些后，还剩4包，每包25本，卖出多少本？
<gadget id="calculator">4 * 25</gadget>
<output>100</output>

<gadget id="calculator">600 - 100</gadget>
<output>500</output>

<result>500</result>