I'm really confused about the concept of short circuits.
I don't understand, why when you short those two voltage sources, 20 and 10 ohm resistors become parallel? What happens with 4, 12, 15 ohm resistors?
At first, I thought that when you short the voltage source, all the resistors that are in parallel, and in series become 0.
However, in the solutions that i was provided, when voltage source (12 V) was replaced by a short circuit, they used voltage divider and included only 10 and 20 ohm resistors. But 12V voltage source is not in parallel with 4 ohm resistor, so why 4 ohm resistor was ignored?
And then, when they short 9V voltage source, 10 and 20 ohm resistors become 0.
Also, I have been watching some tutorials, and when there was a voltage source in series with a resistor, and this voltage source was replaced by a short circuit, the resistor was still there.