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JAMB Physics 2014 Question 38

Question 38

Current Electricity

A circuit has a resistance of 200Ω. The resistance of the circuit can be reduced to 120Ω when

  1. A.an 80Ω resistor is connected to it in series.
  2. B.a 150Ω resistor is connected to it in parallel.
  3. C.a 240Ω resistor is connected to it in series.
  4. D.a 300Ω resistor is connected to it in parallel.
Show the answer and explanation

Answer: D (a 300Ω resistor is connected to it in parallel.)

Explanation

Adding in series always increases resistance, so a parallel resistor is needed: 1/120 = 1/200 + 1/R gives 1/R = 1/300, R = 300 Ω.