Class 31 students donated 90 extra-curricular books, 18 books were given to the kindergarten class, and the rest were distributed to 4 middle classes on average. How many books did each middle class get?
三一班同学捐了90本课外书，送给幼儿园小班18本，剩下的平均分给4个中班，每个中班分到多少本？
<gadget id="calculator">90 - 18</gadget>
<output>72</output>

<gadget id="calculator">72 / 4</gadget>
<output>18</output>

<result>18</result>