The students of Think Academy went hiking one day.  The students walked in a single line, and their speed was $8$km/h.  The teacher wanted to give the head of the team some instructions, so he ran from the end to the front and delivered the instructions (the time spent on delivering the instructions is negligible) and then turned around and ran back to the end.  The running speed of the teacher is 12km/h and he spent a total of $7.2$ minutes from departure to return.  What is the length of the line? 
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