Year 2: Facts to 20 and related facts
Part of the Year 2 adding and subtracting chapter. Chapter overview · whole chapter on one page
Facts to 20 and related facts
Some sums are so well known that we say the answer at once. These are facts to 20.
If you know one fact, you know more. Here is an example.
3 + 4 = 7. So 30 + 40 = 70, because 3 tens and 4 tens make 7 tens.
Key ideas
- Know the facts to 20 so well that you do not count.
- Use a fact you know to find a sum or a take-away.
- If 3 + 4 = 7, then 30 + 40 = 70.
- Check a take-away by adding back: 70 − 30 = 40, so 40 + 30 = 70.
Key words. A fact is a sum that you know. Know the facts to 20 without counting.
Method. Use the way that makes the fact easy to hold in your head.
- Start with the bigger number. 7 is the bigger number, so start there.7
- Make 10. 7 needs 3 more to make 10.7 + 3 = 10
- Add what is left. 3 of the 6 makes 10, so 3 is left.10 + 3 = 13
Check. Take 6 away from 13. You get 7. ✓
Watch out. Do not count from one again. Start at the bigger number.
Key words. Related facts use the same three numbers.
Method. Think of the adding fact that goes with it.
- Look at the take-away. Take 6 away from 14.14 − 6
- Think of the addition that matches. 6 + ? = 14 is the same fact.6 + 8 = 14
- Write the answer. The missing part is 8.8
Check. 8 + 6 = 14. ✓
Watch out. Take away and add are linked. Use the adding fact to check.
Key words. A ten is a bundle of ten ones.
Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.
- Look at the fact 6 + 4 = 10. This is the fact we know.6 + 4 = 10
- Make each number 10 times bigger. 6 becomes 60 and 4 becomes 40.6 tens = 60, 4 tens = 40
- Work out the answer. The answer is 10 tens, so it is 100.60 + 40 = 100
Check. 10 tens is 100. ✓
Watch out. Use the fact you know. Put a zero on each number and on the answer.
Watch out
Watch out: 3 + 4 = 7, so 30 + 40 = 70. The answer is not 37.
Exam tip
Grown-up tip. Ask: “If 3 + 4 = 7, what is 30 + 40?” Let your child answer, then check with 3 bundles of ten and 4 bundles of ten.
Exercise 2A Facts to 20 and related facts
Non-calculatorKey words. A fact is a sum that you know. Know the facts to 20 without counting.
Method. Use the way that makes the fact easy to hold in your head.
- Start with the bigger number. 9 is the bigger number, so start there.9
- Make 10. 9 needs 1 more to make 10.9 + 1 = 10
- Add what is left. 1 of the 5 makes 10, so 4 is left.10 + 4 = 14
Check. Take 5 away from 14. You get 9. ✓
Watch out. Do not count from one again. Start at the bigger number.
Key words. A fact is a sum that you know. Know the facts to 20 without counting.
Method. Use the way that makes the fact easy to hold in your head.
- Start with the bigger number. 8 is the bigger number, so start there.8
- Make 10. 8 needs 2 more to make 10.8 + 2 = 10
- Add what is left. 2 of the 8 makes 10, so 6 is left.10 + 6 = 16
Check. Take 8 away from 16. You get 8. ✓
Watch out. Do not count from one again. Start at the bigger number.
Key words. Related facts use the same three numbers.
Method. Think of the adding fact that goes with it.
- Look at the take-away. Take 7 away from 16.16 − 7
- Think of the addition that matches. 7 + ? = 16 is the same fact.7 + 9 = 16
- Write the answer. The missing part is 9.9
Check. 9 + 7 = 16. ✓
Watch out. Take away and add are linked. Use the adding fact to check.
Method. The box is a number we do not know. Do the opposite to find it.
- Read the sum with the box. The box stands for the missing number.8 + ? = 15
- Do the opposite to find it. To undo adding 8, take 8 away from 15.15 − 8 = 7
- Check it in the sum. Put the number in the box and see if it works.8 + 7 = 15
Check. 8 + 7 = 15. ✓
Watch out. Adding undoes taking away. Taking away undoes adding.
Method. The box is a number we do not know. Do the opposite to find it.
- Read the sum with the box. The box stands for the missing number.? + 6 = 13
- Do the opposite to find it. To undo adding 6, take 6 away from 13.13 − 6 = 7
- Check it in the sum. Put the number in the box and see if it works.7 + 6 = 13
Check. 7 + 6 = 13. ✓
Watch out. Adding undoes taking away. Taking away undoes adding.
Key words. A ten is a bundle of ten ones.
Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.
- Look at the fact 4 + 5 = 9. This is the fact we know.4 + 5 = 9
- Make each number 10 times bigger. 4 becomes 40 and 5 becomes 50.4 tens = 40, 5 tens = 50
- Work out the answer. The answer is 9 tens, so it is 90.40 + 50 = 90
Check. 9 tens is 90. ✓
Watch out. Use the fact you know. Put a zero on each number and on the answer.
Key words. A ten is a bundle of ten ones.
Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.
- Look at the fact 9 − 3 = 6. This is the fact we know.9 − 3 = 6
- Make each number 10 times bigger. 9 becomes 90 and 3 becomes 30.9 tens = 90, 3 tens = 30
- Work out the answer. The answer is 6 tens, so it is 60.90 − 30 = 60
Check. 6 tens is 60. ✓
Watch out. Use the fact you know. Put a zero on each number and on the answer.
Key words. A ten is a bundle of ten ones.
Method. The same fact works with tens. Each number is 10 times bigger, and so is the answer.
- Look at the fact 8 − 5 = 3. This is the fact we know.8 − 5 = 3
- Make each number 10 times bigger. 8 becomes 80 and 5 becomes 50.8 tens = 80, 5 tens = 50
- Work out the answer. The answer is 3 tens, so it is 30.80 − 50 = 30
Check. 3 tens is 30. ✓
Watch out. Use the fact you know. Put a zero on each number and on the answer.
Key words. A fact is a sum that you know. Know the facts to 20 without counting.
Method. Use the way that makes the fact easy to hold in your head.
- Start with the bigger number. 7 is the bigger number, so start there.7
- Make 10. 7 needs 3 more to make 10.7 + 3 = 10
- Add what is left. 3 of the 4 makes 10, so 1 is left.10 + 1 = 11
Check. Take 4 away from 11. You get 7. ✓
Watch out. Do not count from one again. Start at the bigger number.
Key words. Related facts use the same three numbers.
Method. Think of the adding fact that goes with it.
- Look at the take-away. Take 5 away from 12.12 − 5
- Think of the addition that matches. 5 + ? = 12 is the same fact.5 + 7 = 12
- Write the answer. The missing part is 7.7
Check. 7 + 5 = 12. ✓
Watch out. Take away and add are linked. Use the adding fact to check.
Key words. A fact is a sum that you know. Know the facts to 20 without counting.
Method. Use the way that makes the fact easy to hold in your head.
- Start with the bigger number. 9 is the bigger number, so start there.9
- Make 10. 9 needs 1 more to make 10.9 + 1 = 10
- Add what is left. 1 of the 6 makes 10, so 5 is left.10 + 5 = 15
Check. Take 6 away from 15. You get 9. ✓
Watch out. Do not count from one again. Start at the bigger number.
Method. Altogether means add. Add the two amounts.
- Find the first amount. There are 7 books to start.7 books
- Add the second amount. There are 9 more books.7 + 9 = 16
- Write the answer. Altogether means add.16
Check. Take 9 away from 16. You get 7. ✓
Watch out. Read the question again. Did we add or did we take away?
Method. Add the tens, then write the answer in tens.
- Add the tens. 3 tens and 6 tens is 3 + 6 tens.3 + 6 = 9
- Write the answer in tens. So 30 + 60 = 90, which is 9 tens.9 tens = 90
- Write the answer. Yes. 3 + 6 = 9, so 30 + 60 = 90.
Check. Count in tens: 30, 60, 90. ✓
Watch out. Tens add in the same way as ones: 3 + 6 = 9.