A mountain road is downhill from point A to point B, and uphill from point B to point C. The time it takes Xiao Zhang to go from point A to point C via point B is 5 minutes longer than the time it takes to start from point C via point B and return to point A. It is known that Xiao Zhang's uphill speed is downhill (2/3), and the ratio of the distance between AB and BC is 5:7. So, how many hours does it take Xiao Zhang to go back and forth on this mountain road?
一条山路，从A地到B地是下坡路，从B地到C地是上坡路．小张从A地出发经B地到C地所用的时间，比从C地出发经B地回到A地的时间多5分钟．已知小张上坡的速度是下坡的(2/3)，AB两地的路程与BC两地的路程的比是5：7．那么，小张在这条山路上往返一次要多少小时．
<result>2.5</result>