Mental Addition and Subtraction for Year 2 Students

Complete guide covering mental math strategies: counting on, making tens, decomposing numbers, compensation method, and number line strategies for Year 2 learners.

Year 2 Exercises: Problems 1 to 3
1 Counting On Strategy
Problem 1: 23 + 4
Solve mentally using the counting on strategy.
Show your thinking process step by step.
Definition:

Counting On Strategy: Start with the larger number and count up by the smaller number. This is the most efficient method when adding small numbers (1, 2, 3, 4, or 5) and helps develop number sense.

Step-by-step method:
  1. Identify the larger number (23) and the smaller number (4)
  2. Start counting from the larger number: 23
  3. Count up by the smaller number: 24, 25, 26, 27
  4. The final number is the answer: 27
Start
23
Count Up
+4
Result
27
Step 1: Identify numbers

23 is the larger number, 4 is the smaller number

Step 2: Start with larger number

Start counting from 23

Step 3: Count up by smaller number

23 → 24 → 25 → 26 → 27 (count 4 numbers)

Step 4: Record the final number

The last number counted is the answer: 27

23 + 4 = 27
Final answer:

23 + 4 = 27

Applied rules:

• Counting On: Start with the larger number to minimize counting

• Sequential Counting: Add one number at a time

• Small Numbers: Best used when adding 1-5

2 Making Tens Strategy
Problem 2: 38 + 6
Solve mentally using the making tens strategy.
Show how you break apart the numbers to make 10.
Definition:

Making Tens Strategy: Break apart one number to make a ten with the other number, then add the remaining amount. This strategy helps students understand number bonds and makes addition easier.

Original
38 + 6
Make Ten
38 + 2 + 4
Simplify
40 + 4 = 44
Step 1: Identify how many needed to make a ten

From 38, we need 2 more to reach 40 (the next ten)

Step 2: Break apart the second number

Break 6 into 2 + 4 (so we can make 10 with 38)

Step 3: Add to make the ten

38 + 2 = 40

Step 4: Add the remaining amount

40 + 4 = 44

38 + 6 = 44
Final answer:

38 + 6 = 44

Applied rules:

• Ten Complements: Know how many more to make the next ten

• Decomposition: Break apart numbers strategically

• Easy Addition: Adding multiples of 10 is easier

3 Decomposing Numbers Strategy
Problem 3: 24 + 35
Solve mentally using the decomposing numbers strategy.
Show how you separate tens and ones.
Definition:

Decomposing Numbers: Break apart both numbers into tens and ones, add the tens together, add the ones together, then combine the results. This strategy reinforces place value understanding.

Original
24 + 35
Decompose
20+4 + 30+5
Result
50+9 = 59
Step 1: Decompose each number

24 = 20 + 4 (2 tens and 4 ones)

35 = 30 + 5 (3 tens and 5 ones)

Step 2: Add the tens together

20 + 30 = 50

Step 3: Add the ones together

4 + 5 = 9

Step 4: Combine the sums

50 + 9 = 59

24 + 35 = 59
Final answer:

24 + 35 = 59

Applied rules:

• Place Value: Separate tens and ones

• Group Like Terms: Add tens with tens, ones with ones

• Combining: Put the tens and ones back together

Mental Math Strategies for Year 2
Counting On: Start with larger number
Strategy 1
Making Tens
38 + 6 = 38 + 2 + 4 = 40 + 4
Break apart to make 10
Decomposing
24 + 35 = 20+4 + 30+5 = 50+9
Separate tens and ones
Compensation
57 + 8 = 60 + 5 = 65
Round and adjust
Key definitions:

Mental Math: Calculating in your head without paper or tools

Decomposing: Breaking numbers into parts (tens and ones)

Compensation: Adjusting numbers to make calculations easier

Place Value: Understanding that digits represent different values based on their position

Number Bonds: Pairs of numbers that add up to another number (like 7 and 3 bond to make 10)

Complete methodology:
  1. Look at the numbers: Identify which strategy might work best
  2. Choose the method: Counting on, making tens, decomposing, or compensation
  3. Apply the strategy: Follow the steps of the chosen technique
  4. Combine results: Put the pieces back together
  5. Verify: Check your answer by using another method
Tip 1: Always start with the larger number when counting on.
Tip 2: Practice number bonds to 10 every day (pairs that make 10).
Tip 3: Decompose both numbers to add tens and ones separately.
Tip 4: Round difficult numbers to make them easier to work with.
Tip 5: Use visual aids like number lines or drawings when first learning.
Common errors: Forgetting to count the starting number, miscounting, adding instead of subtracting when compensating.
Practice areas: Number bonds to 10, basic addition facts, place value understanding.
Memorization tips: Remember that addition is commutative (order doesn't matter).
Year 2 Exercises: Problems 4 to 5
4 Compensation Method
Problem 4: 57 + 8
Solve mentally using the compensation method.
Show how you round and adjust the numbers.
Definition:

Compensation Method: Round one number to make it easier to work with, then adjust the answer by the amount you changed. This method helps with numbers close to multiples of 10.

Original
57 + 8
Round Up
60 + 5
Result
65
Step 1: Look for numbers near tens

57 is close to 60 (3 away)

Step 2: Round one number up

Add 3 to 57 to make it 60

Step 3: Adjust the other number

Subtract 3 from 8 to keep the total the same: 8 - 3 = 5

Step 4: Add the rounded numbers

60 + 5 = 65

57 + 8 = 65
Final answer:

57 + 8 = 65

Applied rules:

• Compensation: When you add to one number, subtract from the other

• Rounding: Make numbers easier by rounding to nearest ten

• Balancing: Keep the equation balanced by adjusting both sides

5 Mental Subtraction Strategy
Problem 5: 42 - 5
Solve mentally using the counting back strategy.
Show how you make subtraction easier mentally.
Definition:

Mental Subtraction: Use counting back or compensation to subtract without paper. In Year 2, students learn to count back efficiently.

Original
42 - 5
Count Back
42 - 5 = 37
Result
37
Step 1: Start with the larger number

Begin with 42

Step 2: Count back by the smaller number

Count back 5 numbers: 41, 40, 39, 38, 37

Step 3: Record the final number

The number you land on is the answer: 37

Step 4: Verify the result

37 + 5 = 42 ✓

42 - 5 = 37
Final answer:

42 - 5 = 37

Applied rules:

• Counting Back: Move backwards on the number line

• Verification: Check by adding the answer to the subtracted number

• Mental Flexibility: Choose the easiest strategy for the numbers

Year 2 Mental Math: Learning Objectives and Progression
Counting On: 23 + 4 = 23 → 24 → 25 → 26 → 27
Strategy Overview
Key definitions:

Mental Math: Performing calculations in your head without paper or tools

Number Bonds: Pairs of numbers that add up to another number (like 7 and 3 bond to make 10)

Place Value: Understanding that digits represent different values based on their position

Commutative Property: Order doesn't matter in addition (a + b = b + a)

Number Sense: Intuitive understanding of numbers and their relationships

Complete methodology:
  1. Analyze the numbers: Look for patterns, round numbers, or easy combinations
  2. Choose the strategy: Counting on, making tens, decomposing, or compensation
  3. Apply the method: Follow the specific steps for the chosen technique
  4. Verify the result: Check your answer by using another method or reversing the operation
  5. Practice regularly: Repetition builds speed and accuracy
Tip 1: Practice number bonds to 10 daily (pairs that make 10).
Tip 2: When adding, start with the larger number to minimize counting.
Tip 3: Round difficult numbers to the nearest ten for easier calculations.
Tip 4: Always verify your mental math by checking with a different method.
Tip 5: Practice regularly to build speed and accuracy.
Tip 6: Use visual aids like number lines or drawings when first learning.
Common errors: Forgetting to compensate properly, miscounting, confusing addition and subtraction steps.
Key memorizations: Number bonds to 10, doubles facts, place value understanding.
Progression goals: By end of Year 2, students should fluently add and subtract within 100 using mental strategies.
Mental Math Strategies Summary:

• Counting On: Start with larger number, count up by smaller number

• Making Tens: Break numbers to form a ten, then add remainder

• Decomposing: Separate into tens and ones, add separately

• Compensation: Round numbers to make calculation easier, adjust result

• Number Line: Visualize jumps forward or backward on a number line

Year 2 Mental Math: Strategy Effectiveness
Comparing Mental Math Approaches for 36 + 17
Different strategies for solving: 36 + 17
Counting On: 36 → 37 → 38... (17 counts)
Making Tens: 36 + 4 + 13 = 40 + 13 = 53
Decomposing: 30+6 + 10+7 = 40+13 = 53
Compensation: 40 + 13 = 53

Analysis: Different strategies offer varying levels of efficiency depending on the numbers involved.

  • Counting On: Simple but lengthy for larger additions
  • Making Tens: Efficient when one number is close to a ten
  • Decomposing: Systematic approach for any two-digit numbers
  • Compensation: Great for numbers near round figures

Year 2 Questions & Answers

Question: I don't understand when to use making tens versus decomposing numbers. Can you explain the difference?

Answer: Great question! The key difference is in how you manipulate the numbers:

  • Making Tens: You break apart ONE number to combine with the other to make a ten. Example: For 38 + 6, you take 2 from 6 to make 38 into 40, leaving 4. So it becomes 40 + 4 = 44.
  • Decomposing: You break apart BOTH numbers into tens and ones. Example: For 24 + 35, you break 24 into 20+4 and 35 into 30+5, then add 20+30=50 and 4+5=9, finally 50+9=59.

When to use which:

  • Use Making Tens when one number is close to a multiple of 10 (like 38, 59, 87)
  • Use Decomposing when neither number is particularly close to a round number

Both strategies aim to make the math easier by working with friendlier numbers!

Question: Why is it better to start counting on from the bigger number? Doesn't it matter which one I start with?

Answer: Yes, it definitely matters! Starting from the bigger number makes mental math much easier because you have fewer numbers to count.

Example: Let's say you want to solve 7 + 25.

  • If you start from 7 and count on 25: 8, 9, 10, 11, 12... (25 counts!) - This is hard to keep track of!
  • If you start from 25 and count on 7: 26, 27, 28, 29, 30, 31, 32 - Much easier!

Starting with the larger number minimizes the amount of counting you need to do, which reduces the chance of making mistakes and helps you find the answer faster. It's all about efficiency in mental math!

Think of it like taking the shortest path - you want to do the least amount of work to get to your answer.

Question: My child struggles with the compensation method. How can I help them understand when to add versus subtract during compensation?

Answer: The compensation method can be tricky! Here's a simple rule to remember:

  • If you ADD to one number: SUBTRACT the same amount from the other number
  • If you SUBTRACT from one number: ADD the same amount to the other number

Example with addition: 57 + 8

  • I add 3 to 57 to make it 60
  • So I must subtract 3 from 8 to keep the total the same
  • Now I have 60 + 5 = 65

Example with subtraction: 42 - 5

  • I can count back 5 from 42: 41, 40, 39, 38, 37
  • So 42 - 5 = 37

The key concept is keeping the equation balanced - whatever you do to one side, you must do to maintain equality.