The school bought 100 copies of "Ancient Chinese Myths and Stories", 40 copies were left in the library, and the rest were lent to the three classes of the third grade. How many copies did each class borrow on average?
学校买来100本《中国古代神话故事》，图书室留下了40本，其余的借给三年级的3个班，平均每个班借了多少本？
<gadget id="calculator">100 - 40</gadget>
<output>60</output>

<gadget id="calculator">60 / 3</gadget>
<output>20</output>

<result>20</result>