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Solving 3N = 21 for N

Aviation Mechanic General (AMG) practice question 17 of 25 on Mathematics. ACS element AM.I.H.K6 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONTo solve the equation 3N = 21 for N, what operation should be performed on both sides of the equation?

  1. ASubtract 3 from both sides
  2. BDivide both sides by 3
  3. CMultiply both sides by 3
Show the answer and explanation

Correct answer: B. Divide both sides by 3

  • A — incorrect 3N means 3 multiplied by N, so subtraction does not undo it; subtracting 3 would leave 3N − 3 = 18.
  • B — correct Dividing both sides by 3 isolates N: 3N ÷ 3 = 21 ÷ 3, so N = 7.
  • C — incorrect Multiplying both sides by 3 gives 9N = 63 and moves further from isolating N.

BACKGROUND

When solving for a variable, you may add, subtract, multiply or divide the terms in the equation, as long as you do the same thing to both sides of the equals sign, to get the variable alone on one side. In 3N = 21, N is multiplied by 3, so the inverse operation is division by 3: 3N ÷ 3 = 21 ÷ 3, giving N = 7. Similarly, N + 17 = 59 is solved by subtracting 17 from both sides, and N ÷ 5 = 50 by multiplying both sides by 5.

STUDY TIP

Undo whatever is done to the variable with the opposite operation, and apply it to both sides.

KEY POINTS

  • Do the same operation to both sides of the equals sign.
  • 3N = 21: divide both sides by 3, N = 7.
  • Use the inverse of the operation acting on the variable.

SOURCES

  • FAA-H-8083-30B ch03 "Algebraic Rules"
Practice the Aviation Mechanic General (AMG) test →Timed 60-question exam, by ACS area, or review your misses. Free.
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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.