The school is going to distribute exercise books to 21 classes. There are 120 copies per class, and the whole school needs to keep 80 copies as spares. How many exercise books should schools buy?
学校准备发练习本，发给21个班．每班120本，全校还需要留80本作为备用．学校应买多少本练习本？
<gadget id="calculator">120 * 21</gadget>
<output>2_520</output>

<gadget id="calculator">2_520 + 80</gadget>
<output>2_600</output>

<result>2_600</result>