Answer
Cannot be 60 mph.
Work Step by Step
For the average velocity to be $60$ mi/h, the total time needs to be $\frac{120 \text {mi}}{60 \text{mi/h}} = 2 \text{h}$.
In the first 100 miles, the car has already traveled for $\frac{100 \text {mi}}{50 \text{mi/h}} = 2 \text{h}$.
Thus, the car has no time left to complete the final 20 miles to bring the average velocity to 60 mi/h.
