At 7 o'clock in the morning, there is a freight train driving from city A to city B at a speed of 55 kilometers per hour; at 9 o'clock in the morning, another passenger car is driving from city A to city B at a speed of 80 kilometers per hour. The distance between them should not be less than 10 kilometers. Question: When should the trucks stop at the latest to let the passenger cars pass?
上午7时有一列货车以每小时55千米的速度从甲城开往乙城；上午9时又有一列客车以每小时80千米的速度从甲城开往乙城，为了行驶安全，列车间的距离不应少于10千米．问：货车最晚应在什么时刻停车让客车通过？
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