Both the 1st bus and the 2nd bus depart at 7:00, the 1st bus departs every 15 minutes, and the 2nd bus departs every 20 minutes. What is the time when the two buses depart at the same time?
1路和2路专车都是7：00发头班车，1路车每15分钟发一趟，2路车每20分钟发一趟，这两路车再次同时发车的时间是多少．
<result>8</result>