There are 123 copies of "Science and Technology Expo" in the school library. There are 5 classes in the sixth grade, and each class borrows 15 copies. The rest are lent to the 4 classes of the fifth grade, how many books are borrowed by each class on average?
学校图书室一共有123本《科技博览》，六年级5个班，每班借了15本．剩下的借给五年级的4个班，平均每班借多少本？
<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<gadget id="calculator">123 - 75</gadget>
<output>48</output>

<gadget id="calculator">48 / 4</gadget>
<output>12</output>

<result>12</result>