The school plans to distribute exercise books to each class of the third grade. There are 26 classes in total, and each class distributes 150 books. The whole school still needs to keep 280 copies as spares. How many exercise books should schools buy?
学校准备给三年级各班发练习本，共26个班，每班发150本．全校还需要留280本作为备用．学校应买多少本练习本？
<gadget id="calculator">150 * 26</gadget>
<output>3_900</output>

<gadget id="calculator">3_900 + 280</gadget>
<output>4_180</output>

<result>4_180</result>