The school bought some exercise books and distributed them to 5 classes, each class had 20 books, and there were 420 books left. How many exercise books did the school buy?
学校买来一些练习本，分给5个班，每班20本，还剩420本．学校买来多少练习本？
<gadget id="calculator">20 * 5</gadget>
<output>100</output>

<gadget id="calculator">100 + 420</gadget>
<output>520</output>

<result>520</result>