Two cities, $A$ and $B$, at a distance of 50 km, are connected by two separate roads. The speed of any vehicle traveling between the two cities on road 1 is 50 km/hr, while the speed on road 2 is ($80/n$) km/hr, where $n$ is the number of vehicles (including the concerned vehicle). If you travel in a vehicle from $A$ to $B$ on road 1 and come back from $B$ to $A$ on road 2 (where there are already three vehicles plying), your approximate average speed is:
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