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The average speed equals $\frac{total distance}{total time}$. The total time is the time going uphill + the time going downhill. This is $\frac{100}{30}+\frac{50}{60} = \frac{125}{30}=\frac{25}{6}$. So the fraction$\frac{total distance}{total time} = \frac{150}{\frac{25}{6}} = 150/25*6=\boxed{36}$