Subtraction Strategies - First Grade

1. Relate Addition and Subtraction Sentences

Concept: Addition and subtraction are opposite operations. They are related and belong to the same fact family.

Formula: If a + b = c, then c - b = a and c - a = b

Fact Family Example (3, 5, 8):

Addition Sentences:

• 3 + 5 = 8

• 5 + 3 = 8

Subtraction Sentences:

• 8 - 5 = 3

• 8 - 3 = 5

Key Understanding:

• Parts + Parts = Whole

• Whole - Part = Other Part

• All four sentences use the same three numbers!

💡 Tip: Use addition facts to help solve subtraction problems! If you know 4 + 3 = 7, then you know 7 - 3 = 4!

2. Subtract Doubles

Concept: When you subtract a number from its double, the answer is always the original number.

Formula: (n + n) - n = n OR Double - Half = Half

Examples:

6 - 3 = 3 (6 is double of 3)

8 - 4 = 4 (8 is double of 4)

10 - 5 = 5 (10 is double of 5)

14 - 7 = 7 (14 is double of 7)

18 - 9 = 9 (18 is double of 9)

How It Works:

If 7 + 7 = 14, then 14 - 7 must equal 7

Think: "I have two equal groups. If I take one group away, I'm left with one group!"

💡 Tip: Know your doubles facts! They make subtracting doubles super easy!

3. Subtract Multiples of 10

Concept: Subtract numbers like 10, 20, 30, 40, 50, 60, 70, 80, 90 from other multiples of 10.

Formula: (Tens - Tens) × 10 = Answer OR Subtract the tens, then add a zero

Examples:

50 - 20 = 30 (Think: 5 tens - 2 tens = 3 tens)

70 - 40 = 30 (Think: 7 tens - 4 tens = 3 tens)

90 - 30 = 60 (Think: 9 tens - 3 tens = 6 tens)

80 - 50 = 30 (Think: 8 tens - 5 tens = 3 tens)

Step-by-Step Method:

Step 1: Ignore the zeros (60 - 20 becomes 6 - 2)

Step 2: Subtract the digits (6 - 2 = 4)

Step 3: Add the zero back (4 becomes 40)

Answer: 60 - 20 = 40

💡 Tip: Count backwards by tens! Start at 60, count back: 50, 40. You counted back 2 tens!

4. Subtract a Multiple of 10 (From Any Number)

Concept: Subtract multiples of 10 (like 10, 20, 30) from any two-digit number.

Formula: Only the tens place changes; ones place stays the same

Examples:

47 - 20 = 27 (4 tens - 2 tens = 2 tens; 7 ones stay)

68 - 30 = 38 (6 tens - 3 tens = 3 tens; 8 ones stay)

55 - 40 = 15 (5 tens - 4 tens = 1 ten; 5 ones stay)

83 - 50 = 33 (8 tens - 5 tens = 3 tens; 3 ones stay)

Visual Method:

Example: 76 - 30 = ?

• 76 = 7 tens + 6 ones

• Subtract 30 = Subtract 3 tens

• 7 tens - 3 tens = 4 tens

• Keep the 6 ones

• Answer: 4 tens + 6 ones = 46

💡 Tip: The ones digit never changes when subtracting multiples of 10!

Quick Strategy Reference

Fact Families

Use addition to check subtraction

Subtract Doubles

Half the number equals the answer

Multiples of 10

Subtract tens, add zero

From Two-Digit

Ones place stays the same

Practice Problems

Relate Addition & Subtraction:

If 6 + 7 = 13, what is 13 - 7?

Answer: 13 - 7 = 6

Subtract Doubles:

What is 16 - 8?

Answer: 16 - 8 = 8 (16 is double of 8)

Subtract Multiples of 10:

What is 70 - 40?

Answer: 70 - 40 = 30 (7 tens - 4 tens = 3 tens)

Subtract a Multiple of 10:

What is 59 - 30?

Answer: 59 - 30 = 29 (5 tens - 3 tens = 2 tens, keep 9 ones)

Important Math Vocabulary

Fact Family: A group of related addition and subtraction facts using the same numbers
Doubles: Two equal addends (e.g., 5 + 5 = 10)
Multiple of 10: Numbers like 10, 20, 30, 40, 50, etc.
Tens Place: The second digit from the right in a number
Ones Place: The first digit from the right in a number