Addition and Subtraction Within 20 Lesson Plan - Grade 1 Mathematics

Master addition and subtraction within 20: strategies, mental math, and number bonds through these 5 detailed exercises.

Solution: Exercises 1 to 3
1 Basic Addition
Exercise 1
Add: 8 + 5 = ?
Definition:

Addition: Combining two or more numbers to find the total amount

Addition method:
  1. Start with the larger number
  2. Count on from that number
  3. Keep track of how many you count
Expression
8 + 5
Count on
9, 10, 11, 12, 13
Result
13
Step 1: Start with the larger number

Begin counting from 8

Step 2: Count on by the second number

Count 5 more: 9, 10, 11, 12, 13

Step 3: Identify the final number

We land on 13, so 8 + 5 = 13

8 + 5 = 13
Final answer:

8 + 5 = 13

Applied rules:

• Count on strategy: Start with first number and count forward

• Addition: Combining quantities

• Number sequence: Each number is one more than the previous

2 Basic Subtraction
Exercise 2
Subtract: 15 - 7 = ?
Definition:

Subtraction: Taking away one number from another to find the difference

Expression
15 - 7
Count back
14, 13, 12, 11, 10, 9, 8
Result
8
Step 1: Start with the first number

Begin counting from 15

Step 2: Count back by the second number

Count 7 less: 14, 13, 12, 11, 10, 9, 8

Step 3: Identify the final number

We land on 8, so 15 - 7 = 8

15 - 7 = 8
Final answer:

15 - 7 = 8

Applied rules:

• Count back strategy: Start with first number and count backward

• Subtraction: Taking away quantities

• Number sequence: Each number is one less than the previous

3 Number Bonds
Exercise 3
Find the missing number: 9 + ? = 16
Definition:

Number bonds: Pairs of numbers that combine to make a specific total

Problem
9 + ? = 16
Think
16 - 9 = ?
Result
7
Step 1: Understand the relationship

We need to find what number added to 9 equals 16

Step 2: Use subtraction to find the missing part

16 - 9 = 7

Step 3: Verify the answer

9 + 7 = 16 ✓

Step 4: Recognize the number bond

9 and 7 are number bonds that make 16

9 + 7 = 16
Final answer:

The missing number is 7

Applied rules:

• Number bonds: Parts that combine to make a whole

• Inverse operations: Addition and subtraction are opposites

• Part-whole relationship: Understanding how numbers fit together

Rules and methods, laws,...
a + b = c
Basic Addition Formula
Addition
8+5=13
Combine numbers
Subtraction
15-7=8
Take away numbers
Number Bonds
9+7=16
Parts make whole
Key definitions:

Addition: Combining numbers to find the total amount

Subtraction: Taking away numbers to find what remains

Number Bonds: Number pairs that combine to make a specific total

Complete methodology:
  1. Identify the operation: Determine if addition or subtraction
  2. Understand the problem: Know what is being asked
  3. Choose the strategy: Use counting, number bonds, or visual models
  4. Apply the method: Execute the chosen strategy
  5. Verify the result: Check if the answer makes sense
Tip 1: Use physical objects to model addition and subtraction problems.
Tip 2: Practice number bonds to build strong addition and subtraction skills.
Tip 3: Use number lines to visualize addition and subtraction.
Tip 4: Connect math problems to real-world situations.
Common errors: Confusing addition and subtraction, miscounting, forgetting to check answers.
Memory aids: Use songs, rhymes, and visual patterns to remember facts.
Facts to know by heart:

• Addition facts: 1+1=2, 2+2=4, 3+3=6, up to 10+10=20

• Subtraction facts: Related to addition facts (if 3+4=7, then 7-3=4)

• Number bonds to 10: 1+9=10, 2+8=10, 3+7=10, 4+6=10, 5+5=10

• Number bonds to 20: 10+10=20, 11+9=20, 12+8=20, etc.

• Commutative property: a + b = b + a

• Inverse relationship: If a + b = c, then c - a = b

Solution: Exercises 4 to 5
4 Word Problems
Exercise 4
Sarah has 12 apples. She eats 5. How many apples does she have now?
Definition:

Word problems: Real-world situations that require mathematical operations

Given
12 - 5 = ?
Operation
Subtraction
Result
7
Step 1: Identify what we know

Sarah starts with 12 apples and eats 5

Step 2: Determine the operation

"Eats" means we need to subtract

Step 3: Set up the equation

12 - 5 = 7

Step 4: State the answer

Sarah has 7 apples now

Sarah has 7 apples
Final answer:

Sarah has 7 apples

Applied rules:

• Word problem solving: Identify key words that indicate operations

• Subtraction in context: "take away," "eat," "left" often mean subtract

• Real-world connection: Math applies to everyday situations

5 Missing Number Problems
Exercise 5
Find the missing number: ? - 6 = 9
Definition:

Missing number problems: Equations with an unknown value to find

Problem
? - 6 = 9
Think
9 + 6 = ?
Result
15
Step 1: Understand the equation

Some number minus 6 equals 9

Step 2: Use the inverse operation

If ? - 6 = 9, then ? = 9 + 6

Step 3: Calculate the missing number

9 + 6 = 15

Step 4: Verify the answer

15 - 6 = 9 ✓

15 - 6 = 9
Final answer:

The missing number is 15

Applied rules:

• Inverse operations: Addition undoes subtraction

• Missing number strategy: Use opposite operation to find unknown

• Verification: Always check the answer in the original problem

Complete Lesson Plan for Addition and Subtraction Within 20
a + b = c, c - a = b
Addition and Subtraction Relationship
Key definitions:

Addition: Combining two or more numbers to find the total (sum)

Subtraction: Taking one number away from another to find the difference

Number Bonds: Pairs of numbers that combine to make a specific total

Complete methodology:
  1. Problem Identification: Recognize the type of operation needed
  2. Strategy Selection: Choose the most effective method
  3. Execution: Apply the chosen strategy accurately
  4. Verification: Check that the answer is reasonable
Tip 1: Practice counting as the foundation for all operations.
Tip 2: Use visual models like number lines and ten frames.
Tip 3: Connect math to real-life situations for better understanding.
Tip 4: Build fluency through repeated practice and games.
Common errors: Mixing up addition and subtraction, computational mistakes, not checking work.
Teaching strategies: Use manipulatives, visual aids, and gradual progression.
Core facts and relationships:

• Addition and subtraction are inverse operations

• Number bonds to 10: 1+9, 2+8, 3+7, 4+6, 5+5

• Number bonds to 20: 10+10, 11+9, 12+8, 13+7, 14+6, 15+5

• Commutative property: a + b = b + a

• Associative property: (a + b) + c = a + (b + c)

• Identity property: a + 0 = a

• These operations build on counting skills

Addition and Subtraction Patterns: Number Bonds Visualization
Exercise 6: Number Bonds to 20
Visualize number bonds that make 20:
10 + 10 = 20, 11 + 9 = 20, 12 + 8 = 20, 13 + 7 = 20, 14 + 6 = 20
15 + 5 = 20, 16 + 4 = 20, 17 + 3 = 20, 18 + 2 = 20, 19 + 1 = 20

Pattern Analysis: Number bonds to 20 show predictable patterns that help with mental math.

  • As one number increases, the other decreases by the same amount
  • Understanding these patterns builds fluency for addition and subtraction
  • Knowing number bonds helps with missing number problems
  • These relationships support mental math strategies

Questions & Answers

Question: My child is comfortable counting but struggles when we introduce addition and subtraction. How can I bridge this gap?

Answer: Building the bridge from counting to operations requires careful scaffolding:

  • Concrete objects: Use manipulatives like blocks, counters, or toys
  • Counting connection: Emphasize that addition is "counting more" and subtraction is "counting back"
  • Visual models: Use number lines, ten frames, or drawings
  • Gradual introduction: Start with numbers within 5, then 10, then 20

Make sure your child can count objects accurately before moving to abstract symbols. Practice the connection between counting and operations daily!

Question: Why do we need to learn number bonds? Can't I just count everything?

Answer: Great question! Number bonds are like shortcuts that make math faster:

  • Speed: Instead of counting 9 + 7 = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, you instantly know it's 16
  • Foundation: Number bonds help with harder problems later
  • Flexibility: They help with both addition and subtraction
  • Memory: They build strong number sense and mental math skills

Think of number bonds like knowing your home address - it saves time and helps you get to the right answer quickly!

Question: How can I help students understand the relationship between addition and subtraction?

Answer: Help students see addition and subtraction as inverse operations:

  • Fact families: Show how 3 + 4 = 7, 4 + 3 = 7, 7 - 3 = 4, 7 - 4 = 3
  • Physical models: Use objects to demonstrate joining and separating
  • Number lines: Show addition as moving forward and subtraction as moving back
  • Missing number problems: Use equations like ? + 3 = 7 and 7 - 3 = ?

Emphasize that addition and subtraction undo each other - they are opposite operations that work together!