Perimeter of Polygons

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SSAT Middle Level: Quantitative › Perimeter of Polygons

Questions 1 - 10
1

A rectangular garden has a length that is 8 feet more than twice its width. If the width is $$w$$ feet, what is the perimeter of the garden in terms of $$w$$?

$$3w + 8$$ feet

$$6w + 16$$ feet

$$w + 16$$ feet

$$4w + 16$$ feet

$$2w + 8$$ feet

Explanation

When you encounter word problems involving rectangles and perimeter, you need to translate the given relationships into algebraic expressions, then apply the perimeter formula.

Let's break down what we know: the width is $$w$$ feet, and the length is "8 feet more than twice the width." This means the length equals $$2w + 8$$ feet. The perimeter of a rectangle is found using the formula: Perimeter = $$2 \times \text{length} + 2 \times \text{width}$$.

Substituting our expressions: Perimeter = $$2(2w + 8) + 2w = 4w + 16 + 2w = 6w + 16$$ feet.

Looking at the wrong answers: Choice B ($$4w + 16$$) represents a common error where students forget to double the width—they correctly doubled the length expression but only counted the width once. Choice C ($$3w + 8$$) occurs when students mistakenly think perimeter means adding length and width just once, without doubling either dimension. Choice D ($$2w + 8$$) is simply the expression for the length alone, not the perimeter—this happens when students get confused about what the question is asking for.

The correct answer is A: $$6w + 16$$ feet.

Study tip: In rectangle perimeter problems, always write out the formula $$P = 2l + 2w$$ first, then substitute your expressions for length and width. This prevents the common mistake of forgetting to double both dimensions. Also, double-check that your final answer makes sense by testing with a simple value for the variable.

2

The perimeter of a regular hexagon is 72 inches. What is the perimeter of a square whose side length equals the side length of the hexagon?

66 inches

60 inches

48 inches

72 inches

54 inches

Explanation

This question tests your understanding of perimeter and how it relates to different polygons with the same side length.

To find the hexagon's side length, remember that a regular hexagon has 6 equal sides. If the perimeter is 72 inches, then each side is $$72 ÷ 6 = 12$$ inches.

Now you need the perimeter of a square with the same 12-inch side length. Since a square has 4 equal sides, the perimeter is $$12 × 4 = 48$$ inches.

Looking at the wrong answers: Choice B (54 inches) might result from multiplying the hexagon's side length by 4.5, which has no geometric meaning here. Choice C (60 inches) could come from mistakenly thinking a square has 5 sides, leading to $$12 × 5 = 60$$. Choice D (66 inches) doesn't follow from any clear mathematical relationship and might be a distractor based on the original perimeter of 72.

The correct answer is A: 48 inches.

When you encounter polygon perimeter problems, always start by identifying how many sides each shape has. For regular polygons, divide the total perimeter by the number of sides to find individual side length. Then multiply that side length by the number of sides in your target shape. Watch out for answer choices that use incorrect numbers of sides or meaningless mathematical operations – they're designed to catch students who rush through the geometric relationships.

3

A regular pentagon and a regular octagon have the same side length. If the pentagon has a perimeter of 45 centimeters, what is the perimeter of the octagon?

72 centimeters

48 centimeters

40 centimeters

64 centimeters

56 centimeters

Explanation

When you encounter problems about regular polygons with the same side length, focus on the relationship between the number of sides and perimeter. A regular polygon has all sides equal, so perimeter equals the number of sides times the side length.

Let's find the side length of the pentagon first. Since the pentagon has 5 equal sides and a perimeter of 45 centimeters, each side length is $$45 ÷ 5 = 9$$ centimeters.

Now we can find the octagon's perimeter. The octagon has 8 sides, each with the same 9-centimeter length as the pentagon. Therefore, the octagon's perimeter is $$8 × 9 = 72$$ centimeters.

Looking at the wrong answers: Choice B (64 centimeters) might result from incorrectly thinking the side length is 8 centimeters, then multiplying $$8 × 8 = 64$$. Choice C (56 centimeters) could come from using 7 as the side length instead of 9, giving $$8 × 7 = 56$$. Choice D (48 centimeters) might occur if you mistakenly calculated the pentagon's side length as 6 centimeters (perhaps dividing 45 by something other than 5), then computed $$8 × 6 = 48$$.

The correct answer is A) 72 centimeters.

Strategy tip: For regular polygon problems, always identify what information you have and what you need. When polygons share the same side length, use one polygon's known perimeter to find that side length, then apply it to calculate the other polygon's perimeter. Remember: perimeter = number of sides × side length.

4

A trapezoid has parallel sides of length 12 cm and 18 cm, and non-parallel sides of length 8 cm and 10 cm. What is the perimeter of the trapezoid?

48 cm

38 cm

28 cm

30 cm

46 cm

Explanation

When you encounter perimeter problems involving polygons like trapezoids, remember that perimeter is simply the sum of all sides, regardless of the shape's other properties or classifications.

A trapezoid is defined by having exactly one pair of parallel sides, but for perimeter calculations, you treat it like any polygon - just add up every side length. Here, you have four sides: the two parallel sides measuring 12 cm and 18 cm, plus the two non-parallel sides measuring 8 cm and 10 cm.

The perimeter calculation is straightforward: $$12 + 18 + 8 + 10 = 48$$ cm, making choice A correct.

Let's examine why the other answers are wrong. Choice B (46 cm) likely results from a simple arithmetic error, perhaps miscalculating $$12 + 18 = 30$$ instead of $$30$$, then adding $$8 + 10 = 18$$ to get $$48$$, but making an error somewhere in the process. Choice C (38 cm) might come from accidentally using only three sides instead of all four, or from misreading one of the measurements. Choice D (30 cm) represents adding only the parallel sides ($$12 + 18 = 30$$), which reflects a fundamental misunderstanding of what perimeter means.

The key strategy here is recognizing that perimeter problems are usually straightforward addition, regardless of the polygon type. Don't let geometric terminology like "trapezoid," "parallel sides," or "non-parallel sides" distract you from the basic task: add all the side lengths together. Always double-check that you've included every side in your calculation.

5

The perimeter of a rectangle is 56 meters. If the length is 4 meters more than twice the width, what is the width of the rectangle?

10 meters

12 meters

16 meters

20 meters

8 meters

Explanation

When you encounter a rectangle problem with constraints on dimensions, you're dealing with a system of equations. Set up variables and translate the word problem into mathematical relationships.

Let's call the width $$w$$ and the length $$l$$. The problem gives us two key pieces of information: the perimeter is 56 meters, and the length is 4 meters more than twice the width.

From the perimeter formula, we get: $$2l + 2w = 56$$, which simplifies to $$l + w = 28$$.

From the length relationship: $$l = 2w + 4$$.

Substituting the second equation into the first: $$(2w + 4) + w = 28$$. This becomes $$3w + 4 = 28$$, so $$3w = 24$$, giving us $$w = 8$$ meters.

Let's verify: if width is 8, then length is $$2(8) + 4 = 20$$. The perimeter is $$2(20) + 2(8) = 56$$

Choice A (8 meters) is correct. Choice B (10 meters) would give a length of 24 and perimeter of 68 - this comes from incorrectly setting up the length equation as $$l = 2w + 2$$. Choice C (12 meters) results from solving $$l + w = 28$$ and $$l = 2w$$ (forgetting the "+4" part), giving length 24 and perimeter 72. Choice D (16 meters) might come from confusing which dimension should be larger or misreading the constraint.

Always define your variables clearly, write out both constraint equations, and substitute carefully. Double-check by plugging your answer back into the original conditions.

6

Two regular polygons have the same side length of 7 cm. If one is a pentagon and the other is a heptagon (7 sides), what is the difference between their perimeters?

21 cm

49 cm

28 cm

14 cm

35 cm

Explanation

When you encounter polygon problems, remember that perimeter simply means the total distance around the shape, which equals the number of sides times the length of each side.

To find the difference in perimeters, you need to calculate each polygon's perimeter separately. A pentagon has 5 sides, so its perimeter is $$5 \times 7 = 35$$ cm. A heptagon has 7 sides, so its perimeter is $$7 \times 7 = 49$$ cm. The difference is $$49 - 35 = 14$$ cm.

Looking at the wrong answers: Choice B (21 cm) represents three times the side length, which doesn't correspond to any meaningful calculation here. Choice C (28 cm) equals four times the side length—you might get this if you incorrectly calculated the difference in the number of sides (7 - 5 = 2) and then multiplied by some other factor. Choice D (35 cm) is actually the pentagon's complete perimeter, not the difference between the two perimeters. This is a common trap where students calculate one value correctly but forget to complete the final step.

The correct answer is A (14 cm) because it represents the actual difference: 49 - 35 = 14 cm.

For polygon perimeter problems, always use the formula: perimeter = number of sides × side length. When comparing polygons, calculate each perimeter completely before finding the difference. Don't try to take shortcuts with the difference in side counts, as this often leads to trap answers.

7

The perimeter of a square is numerically equal to its area. What is the side length of the square?

16 units

4 units

20 units

12 units

8 units

Explanation

This problem tests your ability to set up equations when two different formulas for the same shape are equal to each other. When you see "numerically equal," you need to write an equation setting the two expressions equal.

For a square with side length $$s$$, the perimeter is $$4s$$ and the area is $$s^2$$. Since these are numerically equal, you can write: $$4s = s^2$$

To solve this equation, rearrange it to standard form: $$s^2 - 4s = 0$$. Factor out $$s$$: $$s(s - 4) = 0$$. This gives you two solutions: $$s = 0$$ or $$s = 4$$. Since a square can't have zero side length, $$s = 4$$.

Let's verify: A square with side length 4 has perimeter $$4 \times 4 = 16$$ and area $$4^2 = 16$$. They're equal, confirming our answer.

Looking at the wrong choices: Choice B (8 units) would give a perimeter of 32 and area of 64 - not equal. Choice C (12 units) would give a perimeter of 48 and area of 144 - not equal. Choice D (16 units) would give a perimeter of 64 and area of 256 - not equal. These wrong answers might tempt you if you confuse which formula is which or make calculation errors.

The correct answer is A) 4 units.

Strategy tip: When two geometric formulas are set equal, always set up the equation algebraically first, then solve. Don't try to guess-and-check with the answer choices - you might make arithmetic errors that lead to wrong conclusions.

8

An isosceles triangle has two sides of length 13 inches each and a perimeter of 35 inches. What is the length of the third side?

11 inches

17 inches

15 inches

13 inches

9 inches

Explanation

When you encounter an isosceles triangle problem, remember that an isosceles triangle has exactly two sides of equal length. The key is using the perimeter formula: perimeter equals the sum of all three sides.

Given information: two sides are 13 inches each, and the total perimeter is 35 inches. To find the third side, set up the equation: $$13 + 13 + \text{third side} = 35$$. This simplifies to $$26 + \text{third side} = 35$$, so the third side equals $$35 - 26 = 9$$ inches.

Let's examine why each answer choice is right or wrong:

A) 9 inches is correct, as shown by our calculation above.

B) 11 inches would give a perimeter of $$13 + 13 + 11 = 37$$ inches, which exceeds the given perimeter of 35 inches.

C) 13 inches might seem tempting since you already know two sides are 13 inches, but this would create an equilateral triangle (all sides equal) with perimeter $$13 + 13 + 13 = 39$$ inches, not 35 inches.

D) 15 inches would result in a perimeter of $$13 + 13 + 15 = 41$$ inches, which is far too large.

Study tip: Always double-check your answer by substituting back into the original conditions. Here, verify that $$13 + 13 + 9 = 35$$ inches, and confirm the triangle is indeed isosceles (exactly two equal sides). This verification step catches calculation errors and ensures you haven't misread the problem.

9

A rectangular garden plot is divided into 4 equal smaller rectangles by one horizontal and one vertical line. If the original rectangle has perimeter 60 feet and length 18 feet, what is the total length of all the dividing lines needed?

18 feet

30 feet

6 feet

12 feet

24 feet

Explanation

When you encounter geometry problems involving dividing shapes, visualize the situation clearly and identify exactly what's being asked. This question tests your ability to find dimensions of a rectangle and understand how dividing lines work.

First, find the width of the original rectangle. Since perimeter equals $$2 \times \text{length} + 2 \times \text{width}$$, you have $$60 = 2(18) + 2w$$, which gives you $$60 = 36 + 2w$$, so $$w = 12$$ feet.

Now picture the rectangle: 18 feet long by 12 feet wide. To divide it into 4 equal smaller rectangles, you need one horizontal line and one vertical line. The horizontal line runs the full length of the rectangle (18 feet), and the vertical line runs the full width of the rectangle (12 feet). Therefore, the total length of dividing lines is $$18 + 12 = 30$$ feet.

Looking at the wrong answers: Choice B (24 feet) might come from incorrectly calculating $$18 + 6$$ if you mistakenly halved the width. Choice C (18 feet) represents only the horizontal dividing line, missing the vertical one entirely. Choice D (12 feet) represents only the vertical dividing line, missing the horizontal one.

The correct answer is A) 30 feet.

Strategy tip: On SSAT geometry problems involving divisions or subdivisions, always draw a quick sketch and clearly identify what measurements you need. Don't forget that dividing lines typically run the full length or width of the original shape, and make sure you account for all required lines.

10

A rhombus has the same perimeter as a rectangle with length 18 meters and width 6 meters. What is the side length of the rhombus?

24 meters

48 meters

18 meters

12 meters

15 meters

Explanation

When you encounter problems comparing perimeters of different shapes, remember that perimeter is simply the total distance around the outside of any shape, regardless of its specific form.

First, let's find the rectangle's perimeter. A rectangle has two lengths and two widths, so: $$P = 2(18) + 2(6) = 36 + 12 = 48$$ meters.

Since the rhombus has the same perimeter as the rectangle, the rhombus also has a perimeter of 48 meters. A rhombus is a special quadrilateral where all four sides are equal in length. If we call each side length $$s$$, then: $$4s = 48$$, which means $$s = 12$$ meters.

Looking at the wrong answers: Choice B (15 meters) would give a perimeter of 60 meters, which is too large. This might tempt students who incorrectly add 18 + 6 + 15 = 39 and think they're close. Choice C (18 meters) would create a perimeter of 72 meters; students might choose this by focusing only on the rectangle's length and ignoring the perimeter calculation entirely. Choice D (24 meters) results in a 96-meter perimeter, exactly double what we need—this could catch students who forget to divide by 4 after finding the correct perimeter.

The correct answer is A (12 meters).

Strategy tip: When comparing perimeters between different shapes, always calculate the known shape's perimeter completely first, then use the properties of the unknown shape to work backwards. Remember that rhombuses have four equal sides, so divide the total perimeter by 4.

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