Puzzle ZNID 

Workings

Hint
It doesn't matter what the distance is, so try a random distance.

Answer
This cannot be done.

It does not matter how far the trip is, let's assume that it is random distance, say 60 miles, and remembering that Time = Distance ÷ Speed.

The morning trip took 60 miles ÷ 30 mph = 2 hours.

To average the required 60 mph, we would have to travel the total distance (of 120 miles) in 2 hours. However, we've already used all of the two hours for the morning trip!

Choosing any random distance would also show that it can't be done.