What is the equivalent resistance of four 60 Ω resistors in parallel?

• A. 60 Ω
• B. 30 Ω
• C. 240 Ω
• D. 15 Ω

Answer: (D)

When resistors are connected in parallel, the voltage across each one is the same and the currents add up. The total or equivalent resistance is found from the sum of the reciprocals: 1/R_eq = 1/R1 + 1/R2 + 1/R3 + 1/R4. If all four resistors have the same resistance, this becomes 1/R_eq = 4/R. So R_eq = R/4. With four 60 Ω resistors, 1/R_eq = 4/60 = 1/15, which gives R_eq = 15 Ω. This value is smaller than any individual resistor, as expected for parallel combinations. The other numbers would correspond to different arrangements: 60 Ω would be if there were effectively no parallel reduction, 30 Ω comes from two 60 Ω resistors in parallel, and 240 Ω comes from adding the four in series.