Math · Plane Geometry
Perimeter of a Rectangle
Medium
Math
Plane Geometry
Question
A rectangle has length 9 and width 3. What is its perimeter?
Answer choices
- 23
- 24
- 25
- 27
- 12
B Correct answer: B) 24
Set up the problem carefully before computing. Perimeter is the sum of all four sides — for a rectangle, twice the length plus twice the width.
Carry the arithmetic through one step at a time, double-checking signs and units. The ACT writes wrong answers to catch the most common slips, so an answer that "looks right" without verification is risky.
Plug the answer back into the original expression as a sanity check whenever the problem allows. If the substitution does not balance, you have made an arithmetic mistake earlier in the work.
The underlying rule
Perimeter = 2(length + width) = 2(9 + 3) = 2 · 12 = 24.
Why each wrong answer is wrong
- A) 23: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- C) 25: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- D) 27: A common arithmetic or sign error leads to this value; carefully redo the calculation.
- E) 12: A common arithmetic or sign error leads to this value; carefully redo the calculation.
Study tip
Rectangle perimeter = 2L + 2W. Rectangle area = L × W. Same numbers, different operation; do not confuse them.
More Medium Plane Geometry
- Area of a TriangleA triangle has base 6 cm and height 5 cm. What is its area in square centimeters?
- Area of a CircleWhat is the area of a circle with radius 4? Express your answer in terms of π.
- Pythagorean TheoremA right triangle has legs of length 8 and 15. What is the length of its hypotenuse?
- Area of a TriangleA triangle has base 12 cm and height 5 cm. What is its area in square centimeters?
- Area of a CircleWhat is the area of a circle with radius 5? Express your answer in terms of π.