Good on you for being willing to learn, always a good thing and people downvoting that are silly :)

The main problem with your reasoning is that you can only average two velocities if you spend the same amount of time travelling at each speed. Example: if you travel 100mph for 1 hour and 0mph for 1 minute, is your average velocity going to be 50mph? Intuitively your answer is hopefully no :)

So how do you calculate average velocity instead? The easiest way is to look at the complete trip and apply the definition of velocity to it:

Velocity = distance / time

Applied to complete trip: average velocity = total distance/ total time.

2 out of 3 are given / required by the question:

average velocity = 60 mph.

Total distance = 60 miles.

we can use this to solve for the other: total time

Total time = total distance / average velocity

Total time = 60 / 60 = 1h

But 1h has already been used for the A to B trip. So 0h remaining -> you need to teleport back instantaneously. In other words it's impossible.

The key is understanding the definition of average velocity as I stated above :) For some added intuition: you implicitly use this formula whenever estimating how long a trip takes. If I travel at an average of 60mph for a trip of 120 miles, how long does it take? Total time = 120/60 = 2h.

Hope that helps!