The teacher took the students for an outing on foot. The scenic spot is 12 kilometers away from the school, and walking is about 4 kilometers per hour. What is the latest they have to leave school to arrive at 9 am?
老师带同学们步行去郊游．景点距学校12千米，步行每小时大约行4千米．要想上午9时到达，他们最晚要几时从学校出发？
<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<result>6</result>