High voltage transmission, transformers and Ohm's law
That 20ohms of the line is in series with the load, not in parallel.

In the first case, the 30ohm load is being driven through the 20ohm series line. The load voltage gets dropped to 30/50 of the sending voltage, so 600v, the remaining 400v is dropped across the line. That's 40% of the input power lost as heat in the line.

In the second case, the 10:1 transformer, when loaded with 30 ohms, presents a 3000 ohm load to the line. At the transformer, the input voltage is loadx10, the input current is load/10, which accounts for the x100 change in apparent resistance. The sending voltage now gets dropped to a ratio of 3000/3020, a factor of 0.993, with the remaining 7v dropped on the line, losing only 0.7% of the input power.