Number bonds to 20
By the end of this chapter you will be able to
- Say the pairs that make 20
- Use bonds to 10 to help find bonds to 20
- Find missing parts in part-whole pictures
- Write related facts for a bond to 20
You should already know
- Number bonds to 10 (Level 1)
- Counting to 100 (Level 2, Chapter 1)
Where this leads
- Adding within 20 (Chapter 5)
- Taking away within 20 (Chapter 6)
Step-by-step practice: maths (Level 2). Small steps, repeated until fluent. Content: Recall the pairs of numbers that make 20, Use bonds to 10 to find bonds to 20, Find missing parts in a part-whole picture with a whole of 20, Write the related addition and subtraction facts for each bond to 20.
Exercise P Do you remember?
Non-calculatorMethod. Find the part that makes 20 with the number we know.
- Start with 5. It is the part we know.5
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 5 = 15
- Write the answer. This is the other part of the bond.15
Check. 5 + 15 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 15. It is the part we know.15
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 15 = 5
- Write the answer. This is the other part of the bond.5
Check. 15 + 5 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 14 away from 10. 14 is 10 and 4, so take away 4 from 10.10 - 4 = 6
- Write the answer. This is the missing part.6
Check. 14 + 6 = 20. Also 10 + 4 + 6 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 4
- Take the known part away from the whole. What is left is the missing part.20 - 4 = 16
- Write the missing part. 16
Check. 4 and 16 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.9 + 11 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 9 = 11
- Write the answer. The other part is the answer.11
Check. 9 + 11 = 20 and 20 - 9 = 11 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Pairs that make 20
A pair of numbers that makes 20 is called a bond to 20. For example, 13 and 7 make 20.
If you know one part, take it away from 20 to find the other part.
Key ideas
- A bond to 20 has two parts that add up to 20.
- The parts can be turned round: 7 and 13 also make 20.
- Take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 13. It is the part we know.13
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 13 = 7
- Write the answer. This is the other part of the bond.7
Check. 13 + 7 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 8. It is the part we know.8
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 8 = 12
- Write the answer. This is the other part of the bond.12
Check. 8 + 12 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 17. It is the part we know.17
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 17 = 3
- Write the answer. This is the other part of the bond.3
Check. 17 + 3 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Watch out
Watch out. The whole is 20, not 10. Do not stop at 10 when you count on.
Exam tip
Grown-up tip. Use 20 counters split into two cups. Ask: “How many in the other cup?”
Exercise 4A Practice: pairs that make 20
Non-calculatorMethod. Find the part that makes 20 with the number we know.
- Start with 9. It is the part we know.9
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 9 = 11
- Write the answer. This is the other part of the bond.11
Check. 9 + 11 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 18. It is the part we know.18
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 18 = 2
- Write the answer. This is the other part of the bond.2
Check. 18 + 2 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 13. It is the part we know.13
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 13 = 7
- Write the answer. This is the other part of the bond.7
Check. 13 + 7 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 16. It is the part we know.16
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 16 = 4
- Write the answer. This is the other part of the bond.4
Check. 16 + 4 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 8. It is the part we know.8
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 8 = 12
- Write the answer. This is the other part of the bond.12
Check. 8 + 12 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 12. It is the part we know.12
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 12 = 8
- Write the answer. This is the other part of the bond.8
Check. 12 + 8 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 15. It is the part we know.15
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 15 = 5
- Write the answer. This is the other part of the bond.5
Check. 15 + 5 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 19. It is the part we know.19
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 19 = 1
- Write the answer. This is the other part of the bond.1
Check. 19 + 1 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 2. It is the part we know.2
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 2 = 18
- Write the answer. This is the other part of the bond.18
Check. 2 + 18 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 6. It is the part we know.6
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 6 = 14
- Write the answer. This is the other part of the bond.14
Check. 6 + 14 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 10. It is the part we know.10
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 10 = 10
- Write the answer. This is the other part of the bond.10
Check. 10 + 10 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 4. It is the part we know.4
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 4 = 16
- Write the answer. This is the other part of the bond.16
Check. 4 + 16 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 14. It is the part we know.14
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 14 = 6
- Write the answer. This is the other part of the bond.6
Check. 14 + 6 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 3. It is the part we know.3
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 3 = 17
- Write the answer. This is the other part of the bond.17
Check. 3 + 17 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 1. It is the part we know.1
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 1 = 19
- Write the answer. This is the other part of the bond.19
Check. 1 + 19 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the part that makes 20 with the number we know.
- Start with 11. It is the part we know.11
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 11 = 9
- Write the answer. This is the other part of the bond.9
Check. 11 + 9 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Using bonds to 10 to find bonds to 20
You already know the bonds to 10. Use them to find bonds to 20.
Take 10 away from 20 first. Then take the rest away from 10. This is quicker than counting on.
Key ideas
- Bonds to 10 help with bonds to 20.
- Take 10 away from 20 first.
- Then take the rest away from 10.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 16 away from 10. 16 is 10 and 6, so take away 6 from 10.10 - 6 = 4
- Write the answer. This is the missing part.4
Check. 16 + 4 = 20. Also 10 + 6 + 4 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 12 away from 10. 12 is 10 and 2, so take away 2 from 10.10 - 2 = 8
- Write the answer. This is the missing part.8
Check. 12 + 8 = 20. Also 10 + 2 + 8 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 19 away from 10. 19 is 10 and 9, so take away 9 from 10.10 - 9 = 1
- Write the answer. This is the missing part.1
Check. 19 + 1 = 20. Also 10 + 9 + 1 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Watch out
Watch out. Take the rest away from 10, not from 20 again. 20 - 10 = 10 is the first step only.
Exam tip
Grown-up tip. Ask: “How many more to make ten?” Then “How many more to make twenty?”
Exercise 4B Practice: bonds to 20 with bonds to 10
Non-calculatorMethod. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 15 away from 10. 15 is 10 and 5, so take away 5 from 10.10 - 5 = 5
- Write the answer. This is the missing part.5
Check. 15 + 5 = 20. Also 10 + 5 + 5 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 11 away from 10. 11 is 10 and 1, so take away 1 from 10.10 - 1 = 9
- Write the answer. This is the missing part.9
Check. 11 + 9 = 20. Also 10 + 1 + 9 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 16 away from 10. 16 is 10 and 6, so take away 6 from 10.10 - 6 = 4
- Write the answer. This is the missing part.4
Check. 16 + 4 = 20. Also 10 + 6 + 4 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 12 away from 10. 12 is 10 and 2, so take away 2 from 10.10 - 2 = 8
- Write the answer. This is the missing part.8
Check. 12 + 8 = 20. Also 10 + 2 + 8 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 19 away from 10. 19 is 10 and 9, so take away 9 from 10.10 - 9 = 1
- Write the answer. This is the missing part.1
Check. 19 + 1 = 20. Also 10 + 9 + 1 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 17 away from 10. 17 is 10 and 7, so take away 7 from 10.10 - 7 = 3
- Write the answer. This is the missing part.3
Check. 17 + 3 = 20. Also 10 + 7 + 3 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 13 away from 10. 13 is 10 and 3, so take away 3 from 10.10 - 3 = 7
- Write the answer. This is the missing part.7
Check. 13 + 7 = 20. Also 10 + 3 + 7 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 14 away from 10. 14 is 10 and 4, so take away 4 from 10.10 - 4 = 6
- Write the answer. This is the missing part.6
Check. 14 + 6 = 20. Also 10 + 4 + 6 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 18 away from 10. 18 is 10 and 8, so take away 8 from 10.10 - 8 = 2
- Write the answer. This is the missing part.2
Check. 18 + 2 = 20. Also 10 + 8 + 2 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Missing parts in bonds to 20
A bond picture has a whole at the top and two parts underneath.
The whole is 20. If one part is missing, take the other part away from 20.
Key ideas
- The whole is 20.
- The two parts add up to 20.
- Take the known part away from 20.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 9
- Take the known part away from the whole. What is left is the missing part.20 - 9 = 11
- Write the missing part. 11
Check. 9 and 11 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 7
- Take the known part away from the whole. What is left is the missing part.20 - 7 = 13
- Write the missing part. 13
Check. 7 and 13 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 15
- Take the known part away from the whole. What is left is the missing part.20 - 15 = 5
- Write the missing part. 5
Check. 15 and 5 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Watch out
Watch out. Do not add the two parts to find the missing part. Take the known part away from 20.
Exam tip
Grown-up tip. Draw a bond picture on paper. Cover one part with a finger and ask your child to say the hidden part.
Exercise 4C Practice: missing parts with a whole of 20
Non-calculatorMethod. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 11
- Take the known part away from the whole. What is left is the missing part.20 - 11 = 9
- Write the missing part. 9
Check. 11 and 9 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 10
- Take the known part away from the whole. What is left is the missing part.20 - 10 = 10
- Write the missing part. 10
Check. 10 and 10 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 12
- Take the known part away from the whole. What is left is the missing part.20 - 12 = 8
- Write the missing part. 8
Check. 12 and 8 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 19
- Take the known part away from the whole. What is left is the missing part.20 - 19 = 1
- Write the missing part. 1
Check. 19 and 1 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 6
- Take the known part away from the whole. What is left is the missing part.20 - 6 = 14
- Write the missing part. 14
Check. 6 and 14 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 9
- Take the known part away from the whole. What is left is the missing part.20 - 9 = 11
- Write the missing part. 11
Check. 9 and 11 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 16
- Take the known part away from the whole. What is left is the missing part.20 - 16 = 4
- Write the missing part. 4
Check. 16 and 4 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 14
- Take the known part away from the whole. What is left is the missing part.20 - 14 = 6
- Write the missing part. 6
Check. 14 and 6 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 18
- Take the known part away from the whole. What is left is the missing part.20 - 18 = 2
- Write the missing part. 2
Check. 18 and 2 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 5
- Take the known part away from the whole. What is left is the missing part.20 - 5 = 15
- Write the missing part. 15
Check. 5 and 15 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 7
- Take the known part away from the whole. What is left is the missing part.20 - 7 = 13
- Write the missing part. 13
Check. 7 and 13 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 2
- Take the known part away from the whole. What is left is the missing part.20 - 2 = 18
- Write the missing part. 18
Check. 2 and 18 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 3
- Take the known part away from the whole. What is left is the missing part.20 - 3 = 17
- Write the missing part. 17
Check. 3 and 17 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 13
- Take the known part away from the whole. What is left is the missing part.20 - 13 = 7
- Write the missing part. 7
Check. 13 and 7 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 1
- Take the known part away from the whole. What is left is the missing part.20 - 1 = 19
- Write the missing part. 19
Check. 1 and 19 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 15
- Take the known part away from the whole. What is left is the missing part.20 - 15 = 5
- Write the missing part. 5
Check. 15 and 5 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Related facts for a bond to 20
One bond gives four facts. For 6 and 14 make 20 there are two additions and two subtractions.
Use the same three numbers in every fact: 6, 14 and 20.
Key ideas
- 6 + 14 = 20 and 14 + 6 = 20.
- 20 - 6 = 14 and 20 - 14 = 6.
- Each fact uses the same three numbers.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.6 + 14 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 6 = 14
- Write the answer. The other part is the answer.14
Check. 6 + 14 = 20 and 20 - 6 = 14 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. Find the missing part of the addition.
- Look at the 14 we know. It is one of the two parts.14
- Count on or take away from 20. The missing part is the rest of 20.20 - 14 = 6
- Write the missing number. 6
Check. 14 + 6 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.11 + 9 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 11 = 9
- Write the answer. The other part is the answer.9
Check. 11 + 9 = 20 and 20 - 11 = 9 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Watch out
Watch out. 20 - 6 is not the same as 6 - 20. Subtract the smaller part from the whole.
Exam tip
Grown-up tip. Write the fact family for each bond on a sticky note. Then cover one number and ask for it.
Exercise 4D Practice: related facts
Non-calculatorMethod. Find the missing part of the addition.
- Look at the 16 we know. It is one of the two parts.16
- Count on or take away from 20. The missing part is the rest of 20.20 - 16 = 4
- Write the missing number. 4
Check. 16 + 4 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. Find the missing part of the addition.
- Look at the 3 we know. It is one of the two parts.3
- Count on or take away from 20. The missing part is the rest of 20.20 - 3 = 17
- Write the missing number. 17
Check. 3 + 17 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.19 + 1 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 19 = 1
- Write the answer. The other part is the answer.1
Check. 19 + 1 = 20 and 20 - 19 = 1 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.11 + 9 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 11 = 9
- Write the answer. The other part is the answer.9
Check. 11 + 9 = 20 and 20 - 11 = 9 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. Find the missing part of the addition.
- Look at the 18 we know. It is one of the two parts.18
- Count on or take away from 20. The missing part is the rest of 20.20 - 18 = 2
- Write the missing number. 2
Check. 18 + 2 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.4 + 16 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 4 = 16
- Write the answer. The other part is the answer.16
Check. 4 + 16 = 20 and 20 - 4 = 16 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.3 + 17 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 3 = 17
- Write the answer. The other part is the answer.17
Check. 3 + 17 = 20 and 20 - 3 = 17 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.16 + 4 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 16 = 4
- Write the answer. The other part is the answer.4
Check. 16 + 4 = 20 and 20 - 16 = 4 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. Find the missing part of the addition.
- Look at the 19 we know. It is one of the two parts.19
- Count on or take away from 20. The missing part is the rest of 20.20 - 19 = 1
- Write the missing number. 1
Check. 19 + 1 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. Find the missing part of the addition.
- Look at the 2 we know. It is one of the two parts.2
- Count on or take away from 20. The missing part is the rest of 20.20 - 2 = 18
- Write the missing number. 18
Check. 2 + 18 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.14 + 6 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 14 = 6
- Write the answer. The other part is the answer.6
Check. 14 + 6 = 20 and 20 - 14 = 6 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. Find the missing part of the addition.
- Look at the 14 we know. It is one of the two parts.14
- Count on or take away from 20. The missing part is the rest of 20.20 - 14 = 6
- Write the missing number. 6
Check. 14 + 6 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. Find the missing part of the addition.
- Look at the 11 we know. It is one of the two parts.11
- Count on or take away from 20. The missing part is the rest of 20.20 - 11 = 9
- Write the missing number. 9
Check. 11 + 9 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.13 + 7 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 13 = 7
- Write the answer. The other part is the answer.7
Check. 13 + 7 = 20 and 20 - 13 = 7 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.6 + 14 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 6 = 14
- Write the answer. The other part is the answer.14
Check. 6 + 14 = 20 and 20 - 6 = 14 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.7 + 13 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 7 = 13
- Write the answer. The other part is the answer.13
Check. 7 + 13 = 20 and 20 - 7 = 13 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Chapter summary
| Skill | Method |
|---|---|
| Pairs that make 20 | Take the known part away from 20 |
| Bonds to 10 help | Take 10 away from 20, then the rest away from 10 |
| Part-whole pictures | The whole is 20. Take the known part away |
| Related facts | Use the same three numbers: two parts and the whole |
Can you…?
- I can say the pairs that make 20
- I can use bonds to 10 to find bonds to 20
- I can find the missing part in a bond picture with a whole of 20
- I can write the related facts for a bond to 20
Key terms
- bond to 20
- two numbers that add up to 20, such as 13 and 7
- whole
- the number we get when the parts are put together
- part
- one of the numbers that makes the whole
- related facts
- the addition and subtraction facts that use the same three numbers
Exercise 4Z Check-up
Non-calculatorMethod. Find the part that makes 20 with the number we know.
- Start with 7. It is the part we know.7
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 7 = 13
- Write the answer. This is the other part of the bond.13
Check. 7 + 13 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the missing part of the addition.
- Look at the 7 we know. It is one of the two parts.7
- Count on or take away from 20. The missing part is the rest of 20.20 - 7 = 13
- Write the missing number. 13
Check. 7 + 13 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 17 away from 10. 17 is 10 and 7, so take away 7 from 10.10 - 7 = 3
- Write the answer. This is the missing part.3
Check. 17 + 3 = 20. Also 10 + 7 + 3 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Find the part that makes 20 with the number we know.
- Start with 18. It is the part we know.18
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 18 = 2
- Write the answer. This is the other part of the bond.2
Check. 18 + 2 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 13 away from 10. 13 is 10 and 3, so take away 3 from 10.10 - 3 = 7
- Write the answer. This is the missing part.7
Check. 13 + 7 = 20. Also 10 + 3 + 7 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 19
- Take the known part away from the whole. What is left is the missing part.20 - 19 = 1
- Write the missing part. 1
Check. 19 and 1 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 2
- Take the known part away from the whole. What is left is the missing part.20 - 2 = 18
- Write the missing part. 18
Check. 2 and 18 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 15 away from 10. 15 is 10 and 5, so take away 5 from 10.10 - 5 = 5
- Write the answer. This is the missing part.5
Check. 15 + 5 = 20. Also 10 + 5 + 5 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 19 away from 10. 19 is 10 and 9, so take away 9 from 10.10 - 9 = 1
- Write the answer. This is the missing part.1
Check. 19 + 1 = 20. Also 10 + 9 + 1 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Find the missing part of the addition.
- Look at the 19 we know. It is one of the two parts.19
- Count on or take away from 20. The missing part is the rest of 20.20 - 19 = 1
- Write the missing number. 1
Check. 19 + 1 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Total 19 marks. Allow about 19 minutes.
Exercise 4X Practice test questions
Non-calculatorMethod. A fact family has an addition and a subtraction with the same numbers.
- Read the addition fact. It has two parts and a whole.5 + 15 = 20
- Use the whole, 20, to make the subtraction. Take one part away from the whole to find the other part.20 - 5 = 15
- Write the answer. The other part is the answer.15
Check. 5 + 15 = 20 and 20 - 5 = 15 use the same numbers.
Watch out. The subtraction takes away a part from the whole, so the answer is the other part.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 14 away from 10. 14 is 10 and 4, so take away 4 from 10.10 - 4 = 6
- Write the answer. This is the missing part.6
Check. 14 + 6 = 20. Also 10 + 4 + 6 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. Find the missing part of the addition.
- Look at the 2 we know. It is one of the two parts.2
- Count on or take away from 20. The missing part is the rest of 20.20 - 2 = 18
- Write the missing number. 18
Check. 2 + 18 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.
Method. Find the part that makes 20 with the number we know.
- Start with 1. It is the part we know.1
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 1 = 19
- Write the answer. This is the other part of the bond.19
Check. 1 + 19 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 12 away from 10. 12 is 10 and 2, so take away 2 from 10.10 - 2 = 8
- Write the answer. This is the missing part.8
Check. 12 + 8 = 20. Also 10 + 2 + 8 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 2
- Take the known part away from the whole. What is left is the missing part.20 - 2 = 18
- Write the missing part. 18
Check. 2 and 18 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. Use a bond to 10 to help. Start from 20 and take away the tens first.
- Take 10 away from 20. That leaves 10.20 - 10 = 10
- Take the rest of 19 away from 10. 19 is 10 and 9, so take away 9 from 10.10 - 9 = 1
- Write the answer. This is the missing part.1
Check. 19 + 1 = 20. Also 10 + 9 + 1 = 20.
Watch out. Use the bond 10 and 10 first. Then the rest is only a small step.
Method. In a part-whole picture, the two parts add up to the whole.
- Look at the whole and the known part. The whole is 20.20 and 8
- Take the known part away from the whole. What is left is the missing part.20 - 8 = 12
- Write the missing part. 12
Check. 8 and 12 make 20. It works.
Watch out. The missing part is never the whole. Find the part by subtracting.
Method. Find the part that makes 20 with the number we know.
- Start with 2. It is the part we know.2
- Find how far it is to 20. Take the known part away from the whole, 20.20 - 2 = 18
- Write the answer. This is the other part of the bond.18
Check. 2 + 18 = 20. It works.
Watch out. The whole is 20, so take the known part away from 20.
Method. Find the missing part of the addition.
- Look at the 1 we know. It is one of the two parts.1
- Count on or take away from 20. The missing part is the rest of 20.20 - 1 = 19
- Write the missing number. 19
Check. 1 + 19 = 20. It works.
Watch out. The whole is 20. The missing part is what is left after you take away the known part.