Solved Exercises on Decomposing Rectangles in Grade 3

Master area and multiplication by learning how to decompose rectangles into smaller parts through these 5 detailed exercises.

Solution: Exercises 1 to 3
1 Basic rectangle decomposition
Exercise 1
Find the area of a rectangle that is 6 units wide and 4 units tall by decomposing it into two smaller rectangles.
Definition:

Decomposing a rectangle: Breaking a large rectangle into smaller rectangles to make finding the total area easier.

Decomposition method:
  1. Draw a line inside the rectangle to separate it into two smaller rectangles
  2. Find the area of each smaller rectangle (length × width)
  3. Add the areas together to get the total area
Original Rectangle
6 × 4 = 24
Decomposed
(4×4) + (2×4)
4×4 = 16
2×4 = 8
Step 1: Decompose the rectangle

We split the 6×4 rectangle into a 4×4 rectangle and a 2×4 rectangle

Step 2: Find area of first part

Area of first rectangle = 4 × 4 = 16 square units

Step 3: Find area of second part

Area of second rectangle = 2 × 4 = 8 square units

Step 4: Add the areas

Total area = 16 + 8 = 24 square units

Total area = 24 square units
Final answer:

The area of the 6×4 rectangle is 24 square units. Decomposing it helped us break the problem into smaller parts: 16 + 8 = 24.

Applied rules:

• Area formula: Area = length × width

• Decomposition: Breaking complex shapes into simpler ones

• Addition principle: Total area = sum of individual areas

2 Decomposing with multiplication
Exercise 2
A rectangle has dimensions 7 units by 5 units. Decompose it into two parts to make multiplication easier. Find the total area.
Definition:

Area model for multiplication: Using rectangles to visualize multiplication by decomposing one factor.

Expression
7 × 5
Decomposed
(5×5) + (2×5)
Result
25 + 10 = 35
5×5 = 25
2×5 = 10
Step 1: Decompose the width

We can break 7 into 5 + 2, so 7×5 becomes (5+2)×5

Step 2: Apply distributive property

(5+2)×5 = (5×5) + (2×5)

Step 3: Calculate each part

5×5 = 25 and 2×5 = 10

Step 4: Add the results

25 + 10 = 35 square units

7 × 5 = 35 square units
Final answer:

The area of the 7×5 rectangle is 35 square units. We decomposed 7 into 5+2 to make the multiplication easier.

Applied rules:

• Distributive property: a(b+c) = ab + ac

• Area formula: Area = length × width

• Decomposition strategy: Break larger numbers into friendlier parts

3 Multiple decomposition
Exercise 3
Find the area of an 8×6 rectangle by decomposing it into four smaller rectangles.
Definition:

Multiple decomposition: Breaking a rectangle into more than two smaller rectangles.

Expression
8 × 6
Decomposed
(4×3) + (4×3) + (4×3) + (4×3)
Result
12 + 12 + 12 + 12 = 48
4×3 = 12
4×3 = 12
4×3 = 12
4×3 = 12
Step 1: Decompose both dimensions

We break 8 into 4+4 and 6 into 3+3, creating four 4×3 rectangles

Step 2: Find area of each small rectangle

Each small rectangle is 4×3 = 12 square units

Step 3: Count the rectangles

There are 4 rectangles, each with area 12

Step 4: Add all areas

12 + 12 + 12 + 12 = 48 square units

8 × 6 = 48 square units
Final answer:

The area of the 8×6 rectangle is 48 square units. We decomposed it into four 4×3 rectangles.

Applied rules:

• Area formula: Area = length × width

• Multiple decomposition: Breaking into more than two parts

• Addition principle: Total area = sum of all parts

Rules and methods, laws,...
Area = length × width
Area Formula
Distributive Property
a(b+c) = ab + ac
Used in decomposition
Area Addition
Total Area = A₁ + A₂ + ... + Aₙ
Sum of parts equals whole
Rectangle Parts
Whole = Part₁ + Part₂
Breaking into smaller rectangles
Key definitions:

Area: The amount of space inside a shape, measured in square units

Decompose: To break apart into smaller pieces

Rectangle: A four-sided shape with four right angles

Complete methodology:
  1. Identify the rectangle: Note its length and width
  2. Choose how to decompose: Decide where to draw lines to make smaller rectangles
  3. Calculate each part: Find the area of each smaller rectangle
  4. Add the areas: Sum all parts to get the total area
Tip 1: Look for "friendly" numbers like 5s and 10s when decomposing.
Tip 2: Draw lines horizontally or vertically to create smaller rectangles.
Tip 3: Check your work by calculating the area directly without decomposition.
Tip 4: Use grid paper to visualize the decomposition clearly.
Common errors: Forgetting to add all parts, miscalculating individual areas, miscounting squares.
Key concepts: Area is additive, rectangles can be broken into smaller rectangles, multiplication connects to area.
Formulas to know by heart:

• Area of rectangle = length × width

• Total area = sum of areas of parts

• Distributive property: a(b+c) = ab + ac

• Area model: (a+b)×c = (a×c) + (b×c)

Solution: Exercises 4 to 5
4 Real-world application
Exercise 4
Sarah has a garden that is 9 feet long and 4 feet wide. She wants to plant flowers in one section that is 5 feet long and vegetables in the rest. What is the area of each section?
Definition:

Real-world application: Using decomposition to solve practical area problems.

Total Garden
9 × 4 = 36 ft²
Flower Section
5 × 4 = 20 ft²
Vegetable Section
4 × 4 = 16 ft²
Flowers: 5×4 = 20 ft²
Vegetables: 4×4 = 16 ft²
Step 1: Understand the problem

Garden is 9×4 feet. Flower section is 5 feet long, vegetable section gets the remaining 4 feet.

Step 2: Find flower section area

Flower section: 5 feet × 4 feet = 20 square feet

Step 3: Find vegetable section area

Remaining length: 9 - 5 = 4 feet. Vegetable section: 4 feet × 4 feet = 16 square feet

Step 4: Verify the total

20 + 16 = 36 square feet, which equals 9×4. ✓

Flower section: 20 ft², Vegetable section: 16 ft²
Final answer:

The flower section is 20 square feet and the vegetable section is 16 square feet. Together they make up the total garden area of 36 square feet.

Applied rules:

• Area formula: Area = length × width

• Subtraction: Finding remaining length

• Addition principle: Parts sum to whole

5 Advanced decomposition
Exercise 5
Decompose a 12×8 rectangle into three parts in two different ways. Show that both methods give the same total area.
Definition:

Multiple decomposition strategies: Same rectangle can be broken differently but still yield the same area.

Method 1
(4×8)+(4×8)+(4×8)
Method 2
(12×3)+(12×3)+(12×2)
Both equal
32+32+32 = 96
Method 1: Split width into 4+4+4
4×8 = 32
4×8 = 32
4×8 = 32
Method 2: Split height into 3+3+2
12×3 = 36
12×3 = 36
12×2 = 24
Step 1: Method 1 - Split width

Break 12 into 4+4+4: (4×8)+(4×8)+(4×8) = 32+32+32 = 96

Step 2: Method 2 - Split height

Break 8 into 3+3+2: (12×3)+(12×3)+(12×2) = 36+36+24 = 96

Step 3: Verify original area

Direct calculation: 12×8 = 96 square units

Step 4: Compare methods

Both methods result in the same total area of 96 square units

Both methods: 96 square units
Final answer:

Method 1: Three 4×8 rectangles = 32+32+32 = 96 sq units. Method 2: One 12×3, one 12×3, one 12×2 = 36+36+24 = 96 sq units. Both methods confirm the total area is 96 square units.

Applied rules:

• Area formula: Area = length × width

• Flexibility: Same area can be calculated multiple ways

• Verification: Different methods should yield same result

Decomposition Laws, Methods, and Key Concepts
(a + b) × c = (a × c) + (b × c)
Distributive Property
Key definitions:

Area: The measure of space inside a 2D shape, expressed in square units

Rectangle: A quadrilateral with four right angles and opposite sides equal

Decompose: To break a shape into smaller, non-overlapping parts

Unit square: A square with sides of length 1 unit

Complete methodology:
  1. Visualize the rectangle: Draw the rectangle and identify its dimensions
  2. Plan the decomposition: Decide where to place dividing lines
  3. Calculate partial areas: Find the area of each resulting rectangle
  4. Sum the areas: Add all partial areas to get the total
  5. Verify: Confirm the sum equals the original area
Tip 1: When decomposing, look for ways to create rectangles with easy-to-multiply dimensions.
Tip 2: Always label dimensions on your drawing to avoid confusion.
Tip 3: The order of decomposition doesn't matter - the total area will always be the same.
Tip 4: Use color coding to distinguish between different parts of the decomposed rectangle.
Important property: Area is additive, meaning the area of a whole is the sum of the areas of its non-overlapping parts.
Connection to multiplication: Decomposing rectangles helps understand the distributive property of multiplication.
Essential formulas and rules:

• Area of rectangle = length × width

• Total area = sum of areas of parts

• Distributive property: a(b+c) = ab + ac

• Area model: (a+b)×c represents a rectangle of width c and length (a+b)

• When decomposing: ensure parts don't overlap and cover the whole shape

Common applications:

• Finding area of irregular shapes by breaking into rectangles

• Solving multiplication problems using area models

• Understanding the distributive property visually

• Calculating areas of rooms, gardens, and other real-world spaces

Questions & Answers

Question: Why do we teach students to decompose rectangles instead of just using the area formula directly? What's the benefit?

Answer: Great question! Teaching decomposition provides multiple benefits beyond just finding area:

  • Conceptual understanding: Students develop a deeper understanding of what area means by seeing it as a collection of unit squares
  • Foundation for multiplication: It visually demonstrates the distributive property (a+b)×c = ac + bc
  • Problem-solving skills: Students learn to break complex problems into manageable parts
  • Mental math: Decomposition makes multiplication easier (e.g., 7×8 becomes 5×8 + 2×8)
  • Preparation for advanced math: This skill is essential for understanding area of complex shapes and algebraic factoring

Rather than just memorizing formulas, students understand why the area formula works.

Question: How do I know where to draw the lines when decomposing a rectangle? Are there rules for this?

Answer: There are several strategies for deciding where to draw decomposition lines:

  • Look for "friendly" numbers: Break larger numbers into parts that are easier to multiply (like 7 into 5+2, or 9 into 5+4)
  • Consider the problem context: If the rectangle represents a room with different sections, follow those boundaries
  • Use halves or quarters: For even numbers, splitting in half often simplifies calculations
  • Follow the distributive property: Think about how you'd break apart a multiplication problem

The key rule is that your decomposition lines must go completely across the rectangle (either horizontally or vertically) to create smaller rectangles.

Remember: the location of lines doesn't change the total area, but choosing strategic locations can make calculations easier!

Question: My child is struggling with keeping track of all the parts when decomposing rectangles. Any tips for organization?

Answer: Organization is crucial for success with rectangle decomposition! Here are some helpful strategies:

  • Label everything: Write dimensions on each part of the decomposed rectangle
  • Use color coding: Color each sub-rectangle differently and keep a key
  • Create a table: List each part with its dimensions and area calculation
  • Work systematically: Calculate areas in order (top to bottom, left to right)
  • Check your work: Add areas as you go and verify against the original area

Here's an example organizational approach:

Part Dimensions Area Calculation Area
Top Left 4×3 4×3 12
Top Right 2×3 2×3 6
Bottom 6×2 6×2 12

This systematic approach helps prevent missing or double-counting parts.