The school bought some exercise books and distributed them to 15 classes, with 64 books in each class, leaving 420 books left. How many exercise books did the school buy in total?
学校买来一些练习本，分给15个班，每个班64本，还剩420本．学校共买来多少本练习本？
<gadget id="calculator">15 * 64</gadget>
<output>960</output>

<gadget id="calculator">960 + 420</gadget>
<output>1_380</output>

<result>1_380</result>