I have been asked the following question:
The resistance, r, is not known in fig(1a), why does its value not affect potential at b?
By Thévenin's theorem, any arbitrary voltage source can be replaced by ideal voltage source with a resistor in series (without affecting the rest of the circuit - rendering those two equivalent). However, I don't think that this is the answer they are looking for, neither I can think of any other way to explain why this resistor does not affect value of \$V_b\$. I have tried to do mesh-analysis, but that failed miserably.
Any other ways?