There are 500 story books in the library. Before the holiday, 137 books were lent to the fourth grade, 186 to the fifth grade, and the rest were lent to 3 classes of the sixth grade. How many books did each class of the sixth grade borrow on average?
图书馆有500本故事书，放假前借给四年级137本，又借给五年级186本，剩下的借给六年级3个班，六年级平均每个班借了多少本？
<gadget id="calculator">500 - 137 - 186</gadget>
<output>177</output>

<gadget id="calculator">177 / 3</gadget>
<output>59</output>

<result>59</result>