What is meant by transmission voltage? I had a question in class that went :

$120$kW of power is generated at a power plant and is then transmitted. The resistance in the transmission lines is $0.4 \Omega$. Calculate the power lost as heat for the following transmission voltages.
a) $240$V
b) $24,000$V

What exactly is meant by the transmission voltages? Are they the voltage drop across the transmission lines or the voltage drop between the output end of the power plant and the receiving end? 
 A: Transmission voltages mean what is the voltage of the live wire coming out of power plant. Two wires come out of the power plant, one is live wire and another wire is neutral/earth ("The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons."Qouted).
There is some voltage given to the live wire (output end),while neutral wire (receiving end) is at 0V. So, the transmission voltage is the voltage difference between output and receiving end.
Now, power plant is generating 120KW of power. If power is generated it has to be stored or consumed, there is nothing about storing in the problem, so all power has to be consumed.
Voltage is constant across the line for particular case and we know
Power consumed(P) = Voltage(V) * Current(I)

So, you can calculate the current in the wire required to consume all the power generated using the above formula.
To calculate power loss in transmission line having resistance R = 0.4 ohm, you can use the following formula
P = I^2 * R

This power loss is in the transmission lines only, and only the leftover power can be used in houses, industries, etc.
Now, you can calculate and compare power loss in transmission lines for the cases and analyse the results.

Here I am considering single phase output from the power plant.

If you have any confusion or need clarification you can comment to this answer.
A: The transmission voltage ist the voltage which is supplied at the beginning, if no current flows it will be the same at the end, but you want to transmit 120kW.
A: The transmission voltage is the (nominal) potential difference between the transmission line and the earth. That is, it's the voltage you'd measure between an active transmission line and the ground, near the generator, assuming there's no other load on the line.
To solve your problem, you need to calculate the current, given the generated power and the transmission voltage, and then use that current and the resistance to calculate the power lost.
