Students in classes A, B, and C are going to visit a school 21 kilometers away from the school. The school has only one car, which can only take students from one class. The speed of the car is 36 kilometers per hour, and the speed of the students on foot It is 4 kilometers per hour. How many hours is the minimum required to make the three classes of students arrive at the destination at the same time?
甲、乙、丙三个班的学生要到离学校21千米的某校参观，学校只有一辆车，只能坐一个班的学生，汽车的速度是36千米/小时，学生步行的速度是4千米/小时，要使三个班的学生同时到达目的地，最少需要多少小时？
<gadget id="calculator">4.2 / 4</gadget>
<output>1.05</output>

<gadget id="calculator">3.36 / 4</gadget>
<output>0.84</output>

<gadget id="calculator">1.05 + 0.84</gadget>
<output>1.89</output>

<result>1.89</result>