Guangdong Beijiang Experimental School currently has 31 classes. After graduating from the third grade of junior high school this year, it plans to expand the enrollment of the first grade of junior high school. After the school starts in September, the school has 36 classes. The number of classes in each grade is the same. How many classes are there in the third grade of junior high school graduating this year? ．
广东北江实验学校现有31个班，今年初三毕业后，准备初一扩招，使得9月份开学后学校有36个班，每个年级班级数相同，则今年毕业的初三级有多少个班．
<gadget id="calculator">36 / 3</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">31 - 24</gadget>
<output>7</output>

<result>7</result>