A person walks from A to B. There are regular buses going back and forth between A and B, and the departure intervals of the two places are equal. He found that every 6 minutes there is a bus going to place A, and every 12 minutes a bus going to place B passes by. Q: How often do buses depart from their respective departure stations?
某人从甲地走往乙地．甲、乙两地之间有定时的公共汽车往返，而且两地发车的间隔都相等．他发现每隔6分钟开过来一辆去甲地的公共汽车，每隔12分钟开过去一辆去乙地的公共汽车．问：公共汽车每隔多少分钟从各自的始发站发车？
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