At Shanghai South Railway Station, train A departs every 2.5 hours, and train B departs every 90 minutes. If they depart at the same time at 6:00 in the morning, then at least how many hours later they will depart at the same time.
在上海南站，A列火车每隔2个半小时发车，B列火车每隔90分钟发车，如果他们早上6：00同时发车，那么至少多少小时后它们又同时发车．
<result>7.5</result>