The fifth grade students used their spare time to clean the windows of the classrooms of Zhengde School. In the first week, they wiped 40% of the windows of the classrooms in the school. In the second week, they wiped the windows of 12 classrooms, and the windows of 6 classrooms were not cleaned. How many classrooms does the school have?
五年级的同学利用课余时间擦正德学校教室的窗户玻璃，第一周擦了全校教室窗户玻璃的40%，第二周擦了12间教室窗户玻璃，还剩6间教室窗户玻璃没有擦，正德学校一共有多少间教室？
<gadget id="calculator">12 + 6</gadget>
<output>18</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">18 / (3/5)</gadget>
<output>30</output>

<result>30</result>