Your question contains one very common misconception, and that is that large amounts of current flowing through ground is normal.
In this diagram, notice the lack of a particular path for current flow into or out of ground. As long as everything remains balanced and a power line isn't touching the earth or a tree, there is no path for current to travel through ground (except some minor leakages).
Here is a wye connected four wire system. Notice how the ground is connected directly to the neutral point in the center, and the neutral (N) line has no coil. In this system, the neutral line is the return path for all single phase loads - that is, anything that is connected from a phase to the neutral. Phase to phase connections are still available, of course. But current flow through ground is still not normal.
Which is a great thing - if you put a current transformer on the ground connection, you now have a high reliability mechanism to detect if the system is grounded. This is a standard feature on the power grid.
Now, lets say a high voltage power line has been broken and is laying on the ground. At 500 KV, there is definitely going to be some current flow through the earth. And as with all current flow, voltage drops when current flows across a resistance. Starting from the 500 KV at the end of the line, and reaching zero back at the nearest system ground connection, means that there can be a huge difference between one foot and the other in that vicinity. In the industry, we call this step voltage differential, and it can be lethal. It's the reason why you may have heard that you need to shuffle away from lightning strikes and power lines, rather than walk; that keeps your feet close to each other and prevents a current flow from leg to leg.
On the off chance that a line has been downed and grounded, the current flow will radiate away from the point of earth contact to whatever path is most favorable for current flow. If the topsoil is recently wet, it will tend to stay there. If everything around is fairly dry, there may not actually be much current flow at all, and it will radiate in nearly every direction as the voltage charges the ground. It will flow through ground water, if it can get there.
As far as the difference between AC and DC grounding events, the physical characteristics of DC make it more likely that it does not find a good path back to the nearest system ground, which makes it more likely to have a potentially lethal step voltage differential.
Yes, 60v is what you would see at the end of the quarter-wavelength transmission line. This is due to the multiple reflections back and forth from the open at the end of the transmission line and the impedance mismatch at the input (300-ohm to 50-ohm). This effectively is creating a standing wave giving you 300/50 = 6 multiplication in voltage at the end of the quarter-wavelength and 0V at the input after a few cycles.
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I find that the concept of sending/receiving end voltage is somewhat meaningless in most cases. Actual transmission lines are connected in complex networks with distributed impedance (the line itself) and point impedance (loads tapped off) all over the network. There is no set beginning or end.
In the real world you have to keep all points on the line within the stated tolerance of the nominal voltage. Typically that's +/-10%. At lower, distribution level voltages where the source impedance is higher, they will often times use voltage regulators or on-load tap changers to maintain the bus voltage dynamically.
But for your text book question I think that they want the receiving end voltage to be the nominal 220kV and the sending end voltage would be chosen to allow for drop.