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U.1 Addition and subtraction sentences up to 20

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Understanding Addition and Subtraction up to 20

Addition means putting numbers together to make a total. Subtraction means taking away from a number to see what is left. We can work with numbers up to 20 to build a strong foundation for math.

Examples:
  • 12 + 5 = 17 means twelve and five together make seventeen.
  • 18 − 7 = 11 means if we take seven away from eighteen, eleven are left.
  • 10 + 0 = 10 shows that adding zero keeps the number the same.
Helpful Hint

Think of addition as “putting together” and subtraction as “taking away.” Use numbers up to 20 to practice fluency.

Completing Number Sentences

Sometimes a number sentence has a missing part. We can use what we know about addition and subtraction facts to find the missing number and make the sentence true.

Examples:
  • 15 + ? = 20. Since 15 needs 5 more to make 20, the missing number is 5.
  • ? − 6 = 9. Think: “What number minus 6 equals 9?” The answer is 15.
  • 14 − ? = 8. The missing number is 6.
Helpful Hint

Use known facts and number pairs to help solve for the missing part. Check your work by trying the opposite operation.

Using Objects and Drawings

We can use counters, cubes, or drawings to show addition and subtraction. This helps us see the numbers clearly and understand how they work together.

Examples:
  • Put 9 red counters and 4 blue counters together. Count them all: 9 + 4 = 13.
  • Start with 16 toy cars. Take away 7. Now there are 16 − 7 = 9 cars left.
Helpful Hint

Draw simple pictures or use real objects to solve problems. This makes it easier to check your answers.

Using a Number Line

A number line is a useful tool for solving addition and subtraction problems. Move forward to add and move backward to subtract.

Examples:
  • Start at 7. Move forward 8 steps. You land on 15. So, 7 + 8 = 15.
  • Start at 19. Move back 6 steps. You land on 13. So, 19 − 6 = 13.
Helpful Hint

Always move to the right on the number line when adding and to the left when subtracting.

Using Fact Families

Fact families show how addition and subtraction sentences are related. They use the same three numbers in four different ways.

Examples:
  • Numbers: 8, 12, 20
  • 8 + 12 = 20
  • 12 + 8 = 20
  • 20 − 8 = 12
  • 20 − 12 = 8
Helpful Hint

Learning fact families helps you remember both addition and subtraction facts quickly.

Math in Everyday Life

Addition and subtraction are part of daily activities when we count, share, compare, or find out how many are left.

Real-Life Examples:
  • You have 11 pencils and your friend gives you 6 more. Now you have 11 + 6 = 17 pencils.
  • There are 20 chairs in a room. 8 are taken. Now there are 20 − 8 = 12 chairs left.
  • You read 7 pages in the morning and 9 pages in the afternoon. That’s 7 + 9 = 16 pages in all.
Try This

Practice by adding and subtracting when you share snacks, count toys, or compare groups at home or school.