9 students are responsible for classroom hygiene, and 3 people are required to participate in each cleaning. If any two students can only clean together once, how many cleaning times can be arranged at most.
9名同学负责教室卫生，每次打扫卫生需要3人参加，如果任意两名同学都只能在一起打扫一次卫生，那么最多能安排打扫多少次卫生．
<gadget id="calculator">36 / 3</gadget>
<output>12</output>

<result>12</result>