At 7 o'clock in the morning, car A departs from the station to place A. After 30 minutes, car B departs from the station and goes to place B in the opposite direction at the same speed. Car B had just left for 20 minutes, the squad leader found out that due to negligence, he had given the driver of car A the letter asking the driver of car B to take him to place B by mistake. So, the squad leader immediately set off by car. The speed of the car is twice that of the car. How many hours did it take when the squad leader finished handling the matter and returned to the station?
上午7时，甲车从车站出发前往A地．30分钟后，乙车从车站出发用同样的速度开往相反方向的B地．乙车刚开出20分钟，班长发现，由于一时疏忽，把今天请乙车驾驶员带到B地的信错交给了甲车驾驶员．于是，班长立即乘小车出发．小车的速度是甲车的2倍．当班长处理完这件事返回车站时，一共用了几小时？
<result>8.5</result>