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Total resistance of two parallel resistors

Aviation Mechanic General (AMG) practice question 1 of 41 on Fundamentals of Electricity and Electronics. ACS element AM.I.A.K12 · difficulty: applied. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONTwo resistors of 20 ohms and 30 ohms are connected in parallel. What is the total resistance of the combination?

  1. A12 ohms
  2. B50 ohms
  3. C25 ohms, the average of the two values
Show the answer and explanation

Correct answer: A. 12 ohms

  • A — correct For two resistors in parallel, RT equals the product divided by the sum: (20 x 30) / (20 + 30) = 600 / 50 = 12 ohms.
  • B — incorrect 50 ohms is the sum of the two values, which would be correct only if they were connected in series.
  • C — incorrect Averaging is not a valid method; the total parallel resistance must be less than the smallest branch resistance.

BACKGROUND

In a parallel circuit more than one path is provided for current, so the total resistance is less than that of any single branch. The general formula is 1/RT = 1/R1 + 1/R2 + 1/R3 and so on. For the common case of two resistors, this simplifies to RT = (R1 x R2) / (R1 + R2), the product divided by the sum. The handbook example with 500 ohms and 400 ohms gives 200,000 / 900 = 222.22 ohms. Any number of parallel resistors can be reduced by working with pairs.

STUDY TIP

Check every parallel answer against the rule that the total must be less than the smallest resistor; it eliminates most wrong choices instantly.

KEY POINTS

  • Two parallel resistors: RT = product over sum.
  • (20 x 30) / (20 + 30) = 12 ohms.
  • Total parallel resistance is always less than the smallest branch.

SOURCES

  • FAA-H-8083-30B ch12 "Parallel DC Circuits" > "Two Resistors in Parallel"
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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.