Subtraction Problems Within 20 - Grade 1 Math

Learn subtraction problems within 20 through visual examples and exercises designed for young learners.

Subtraction Problems Within 20
1 Basic Subtraction
Problem 1
Tom has 15 apples. He eats 6 apples. How many apples does Tom have left?
Definition:

Subtraction within 20: Taking away some items from a group where the answer is 20 or less.

Subtraction method:
  1. Start with the total amount
  2. Take away the amount you're removing
  3. Count what's left
  4. Write the answer
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Step 1: Start with total

Tom had 15 apples

Step 2: Take away eaten apples

He ate 6 apples

Step 3: Count remaining

Count the apples that weren't crossed out: 9

Step 4: Write the answer

15 - 6 = 9

Tom has 9 apples left
Final answer:

Tom has 9 apples left

Subtraction rule:

• Start with total: Begin with the full amount

• Take away: Remove the specified amount

• Count remainder: See what's left

2 Visual Subtraction
Problem 2
There were 18 balloons. 9 balloons flew away. How many balloons are left?
Definition:

Visual subtraction: Using pictures to see what happens when items are removed.

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Step 1: Count total balloons

There were 18 balloons

Step 2: Identify what flew away

9 balloons flew away

Step 3: Count remaining balloons

Count the balloons that didn't fly away: 9

Step 4: Write the subtraction sentence

18 - 9 = 9

9 balloons are left
Final answer:

9 balloons are left

Subtraction rule:

• Total minus taken away equals remaining

• Visual cues help understand the process

• Always check that your answer makes sense

3 Subtraction Word Problem
Problem 3
16 children were playing in the park. 7 children went home. How many children are still playing?
Definition:

Word problem subtraction: Using subtraction to solve real-life scenarios.

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Step 1: Identify the starting number

16 children were playing

Step 2: Identify what's being subtracted

7 children went home

Step 3: Perform the subtraction

16 - 7 = 9

Step 4: State the answer

9 children are still playing

9 children are still playing
Final answer:

9 children are still playing

Subtraction rule:

• Word problems require careful reading

• Identify what's being taken away

• Always state the answer in context

Subtraction Concepts Within 20
Minuend - Subtrahend = Difference
Subtraction Formula
Minuend
≤ 20
Start with
Subtrahend
≤ Minuend
Take away
Difference
≤ 20
What's left
20-1=19
Max
18-5=13
Large
15-7=8
Medium
12-3=9
Small
Minus Sign (-): Means "take away" or "subtract."
Decreasing: Subtraction always results in a smaller number.
Checking: Add the answer back to check your work.
Subtraction Principles Within 20
4 Subtraction Steps
Key Definitions:

Subtraction: The operation of taking away one number from another

Within 20: The starting number, the number taken away, and the result are all 20 or less

Minuend: The number you start with (must be ≤ 20)

Start With
≤ 20
Take Away
≤ Start
Left With
≤ 20
Step 1: Identify the total

Find the number you start with (must be 20 or less)

Step 2: Identify what's taken away

Find the number being subtracted (can't be more than starting number)

Step 3: Perform the subtraction

Count backward or take away items

Subtraction within 20 finds how many are left after taking some away
Key concept:

Subtraction is the opposite of addition and results in a smaller number

5 Common Subtraction Mistakes
Common errors:

Mistake 1: Subtracting the larger number from the smaller number

Mistake 2: Counting incorrectly when taking away

Mistake 3: Forgetting to check the answer

Correct
Larger - Smaller
Incorrect
Smaller - Larger
Step 1: Always subtract from the larger amount

Start with the total, take away the part

Step 2: Count carefully

Make sure you're taking away the right amount

Step 3: Verify your answer

Add the answer back to check: 9 + 6 = 15

Always verify: Does the answer make sense?
Remember:

You can't take away more than you have!

Comprehensive Subtraction Within 20 Summary
A - B = C (where A, B, C ≤ 20)
Subtraction Formula
Key definitions:

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

Within 20: All numbers involved in the problem are 20 or less

Minuend: The number being subtracted from (A in A-B=C)

Subtraction method:
  1. Identify: Find the total amount (minuend) - must be ≤ 20
  2. Determine: Find what's being taken away (subtrahend) - must be ≤ minuend
  3. Subtract: Take away the subtrahend from the minuend
  4. Count: Find how many are left (difference) - must be ≤ 20
  5. Verify: Check by adding the difference to the subtrahend
Tip 1: Use your fingers to count when subtracting.
Tip 2: Draw circles and cross out the ones being taken away.
Tip 3: Count backward from the starting number.
Tip 4: Always check your answer by adding back.
Common errors: Subtracting in wrong order, miscounting, not verifying answers.
Exam preparation: Practice visual subtraction and word problems.
Subtraction rules:

• Subtraction always results in a smaller number

• The minuend must be larger than or equal to the subtrahend

• You can check subtraction by adding: difference + subtrahend = minuend

• The minus sign (-) indicates subtraction

• All numbers in the problem are 20 or less

Questions & Answers

Question: Why do we learn subtraction within 20? What's different from within 10?

Answer: We learn subtraction within 20 to build on what we learned with numbers up to 10!

  • It helps us work with bigger numbers
  • We practice the same subtraction skills but with larger numbers
  • It prepares us for even bigger numbers later
  • We can solve more complex real-life problems

Think of it like moving from walking to jogging!

The same rules apply, but now we can work with numbers up to 20!

Question: How can I help my child practice subtraction within 20?

Answer: Use everyday objects and activities:

  • Use blocks, coins, or toys to physically take away items
  • Practice with snacks: "You have 16 crackers, eat 7, how many left?"
  • Count backward from a number
  • Use fingers to count and take away
  • Play games like "I have 18, I take away 5, how many do I have?"

Start with smaller numbers within 20 and gradually increase difficulty.

Make it fun and engaging for better learning!

Question: How do I know if my subtraction answer is right?

Answer: There are several ways to check your subtraction answer:

  • Add your answer to the number you took away: if 15 - 6 = 9, then 9 + 6 = 15
  • Count backward: Start at the big number and count back the amount you're subtracting
  • Use objects: Actually take away the items and count what's left
  • Make sure your answer is smaller than the starting number

For example: If you have 16 and take away 7, your answer should be less than 16!

Always double-check to make sure your answer makes sense!