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Total resistance of a series-parallel circuit

Aviation Mechanic General (AMG) practice question 13 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K26 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a series-parallel circuit, a 1-kilohm resistor is in series with a parallel pair of 2 kilohms and 3 kilohms. What is the total resistance of the circuit?

  1. A6 kilohms
  2. B2.2 kilohms
  3. C3.5 kilohms, using the average of the pair
Show the answer and explanation

Correct answer: B. 2.2 kilohms

  • A — incorrect 6 kilohms is the simple sum of all three resistors, which treats the parallel pair as if it were in series.
  • B — correct First reduce the parallel pair: (2 k x 3 k) / (2 k + 3 k) = 6,000 k / 5 k = 1.2 kilohms. Then add the series resistor: 1 k + 1.2 k = 2.2 kilohms.
  • C — incorrect Averaging parallel resistors is not valid; the equivalent of a parallel pair is the product divided by the sum, which is less than either resistor.

BACKGROUND

Most circuits a technician encounters are series-parallel combinations rather than pure series or pure parallel circuits. The same rules used for series and parallel circuits are applied step by step. The first step is to isolate any parallel group and reduce it to a single equivalent resistor using the parallel formula; the circuit then becomes a simple series string whose resistances are added. The handbook example reduces 2 kilohms in parallel with 3 kilohms to 1.2 kilohms, adds the 1-kilohm series resistor to get RT = 2.2 kilohms, and then finds the total current from a 24-volt source as 10.9 mA.

STUDY TIP

Always collapse the parallel groups first; once the circuit is a single series loop the arithmetic is just addition.

KEY POINTS

  • Reduce each parallel group to an equivalent resistor first.
  • 2 k in parallel with 3 k equals 1.2 kilohms.
  • 1 k + 1.2 k gives a total of 2.2 kilohms.

SOURCES

  • FAA-H-8083-30B ch12 "Series-Parallel DC Circuits" > "Determining the Total Resistance" (Figure 12-102 example)
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.