The school bought 800 science and technology books and distributed them to 25 classes in each grade, with 20 books in each class. How many books are left?
学校买来800本科技书，分给各年级共25个班，每班20本．还剩多少本？
<gadget id="calculator">20 * 25</gadget>
<output>500</output>

<gadget id="calculator">800 - 500</gadget>
<output>300</output>

<result>300</result>