school organizes students to go to a summer camp 100 kilometers away. The car can only take half of the students, and the other half walks. 4 kilometers, the car travels 20 kilometers per hour, so that everyone arrives at 5 pm at the same time, when do you need to start?
某学校组织学生到100千米以外的夏令营去，汽车只能坐一半人，另一半人步行，先坐车的人在途中步行，汽车则立即回去接先步行的一半人，已知步行每小时走4千米，汽车每小时走20千米，要使大家下午5点同时到达，问需何时出发？
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