Answer
It is not possible to achieve an average speed of 30 mi/h for the trip.
Work Step by Step
Assume the speed for the descent is $v$, the time the car take for the first mile is
$t1=1/15$ and the time for the second mile is $t2=1/v$.
The average speed can be calculate as $2/(t1+t2)=30$ which gives
$\frac{1}{15}+\frac{1}{v}=\frac{2}{30}$ and by solving this equation we obtain $\frac{1}{v}=0$
which means that it is not possible to achieve an average speed of 30 mi/h for the trip.
