While traveling along and against the traffic stream, a moving observer measured the relative flows as 50 vehicles/hr and 200 vehicles/hr, respectively. The average speeds of the moving observer while traveling along and against the stream are $20 \mathrm{~km} / \mathrm{hr}$ and $30 \mathrm{~km} / \mathrm{hr},$ respectively. The density of the traffic stream (expressed in vehicles/km) is

While traveling along and against the traffic stream, a moving observer measured the relative flows as 50 vehicles/hr and 200 vehicles/hr, respectively. The average speeds of the moving observer while traveling along and against the stream are $20 \mathrm{~km} / \mathrm{hr}$ and $30 \mathrm{~km} / \mathrm{hr},$ respectively. The density of the traffic stream (expressed in vehicles/km) is

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