Answer: 60 Ω.
- A 15 Ω
- B 40 Ω
- C 45 Ω
- D 60 Ω
Correct answer: D. 60 Ω
Explanation: Balance condition P/Q = R/S → S = R·Q/P = 30·20/10 = 60 Ω.
In series, the same current flows through both resistors; in parallel, the current splits between branches and the voltage across each resistor is the same.
Concept context
Ohm's law, resistors, Kirchhoff's laws, Wheatstone bridge, and cells.