Answer
The answer does not exist.
Work Step by Step
Because: if the total time for the trip is $\frac{2 mi}{30 mi/h}$, which is equal to $\frac{1mi}{15mi/h}$, and the equation for the total time of the trip is $t_{1} + t_{2} = t_{3}$, then there is no time for the car travelling downhill, since $t_{2} = t_{3} - t_{1} =\frac{1mi}{15mi/h} - \frac{1mi}{15mi/h}$ . Based on the equation: $speed = \frac{distance}{time}$ .
