Step-by-step Maths Level 1: Number sentences and checking
Part of the Step-by-step Maths Level 1 number stories within 20 chapter. Chapter overview · whole chapter on one page
Number sentences and checking
A number sentence is a sum with the answer, like 7 + 5 = 12.
To check an answer, do the inverse (the opposite). Add undoes take away. Take away undoes add.
Key ideas
- Write the start, the sign, the number, then the answer.
- Use + for more and − for take away.
- Check by doing the opposite: 12 − 5 = 7 is right because 7 + 5 = 12.
Method. Use add for more. Write the start, the sign, the number added and the answer.
- Write the start. Sam starts with 7 cards.7
- Write the sign. More means add.7 +
- Write the rest and the answer. He gets 4 more, so the total is 11.7 + 4 = 11
Check. Check: 11 − 4 = 7. It is right.
Watch out. The equals sign shows the total after the sign.
Method. Do the opposite. Add the number taken away to the answer.
- Look at the subtraction. 13 minus 4 is 8.13 − 4 = 8
- Do the inverse. Add 4 to 8.8 + 4 = 12
- Compare with the start. We needed 13, not 12.12 is not 13
- Write the answer. No
Check. The right answer is 13 − 4 = 9, since 9 + 4 = 13.
Watch out. If the inverse does not give the start, the answer is wrong.
Method. Use take away for fewer. Write the start, the sign, the number taken away and the answer.
- Write the start. There are 15 bees.15
- Write the sign. Fly away means take away.15 −
- Write the rest and the answer. Six fly away, so 9 are left.15 − 6 = 9
Check. Check: 9 + 6 = 15. It is right.
Watch out. Use a minus sign for take away.
Watch out
Watch out. Put the numbers in the right order. 13 − 4 is not the same as 4 − 13.
Exam tip
Grown-up tip. Ask your child to say the story, then write the sum. Check together: “If we add back, do we get the start?”
Exercise 9D Number sentences and checking
Non-calculatorMethod. Use add for more. Write the start, the sign, the number added and the answer.
- Write the start. Amira starts with 12.12
- Write the sign. More means add, so use a plus sign.12 +
- Write the rest and the answer. 5 more are found.12 + 5 = 17
- Check. 12 + 5 = 17
Check. Check with a subtraction: 17 − 5 = 12. ✓
Watch out. Write the sign, then the total after the equals sign.
Method. Use take away for fewer. Write the start, the sign, the number taken away and the answer.
- Write the start. There are 15 bees.15
- Write the sign. Fly away means take away, so use a minus sign.15 −
- Write the rest and the answer. 4 fly away.15 − 4 = 11
- Check. 15 − 4 = 11
Check. Check with addition: 11 + 4 = 15. ✓
Watch out. The minus sign is for taking away. The equals sign shows the answer.
Method. Use add for more. Write the start, the sign, the number added and the answer.
- Write the start. Priya starts with 12.12
- Write the sign. More means add, so use a plus sign.12 +
- Write the rest and the answer. 2 more are found.12 + 2 = 14
- Check. 12 + 2 = 14
Check. Check with a subtraction: 14 − 2 = 12. ✓
Watch out. Write the sign, then the total after the equals sign.
Method. Use take away for fewer. Write the start, the sign, the number taken away and the answer.
- Write the start. There are 8 bees.8
- Write the sign. Fly away means take away, so use a minus sign.8 −
- Write the rest and the answer. 6 fly away.8 − 6 = 2
- Check. 8 − 6 = 2
Check. Check with addition: 2 + 6 = 8. ✓
Watch out. The minus sign is for taking away. The equals sign shows the answer.
Method. Use add for more. Write the start, the sign, the number added and the answer.
- Write the start. Zainab starts with 12.12
- Write the sign. More means add, so use a plus sign.12 +
- Write the rest and the answer. 4 more are found.12 + 4 = 16
- Check. 12 + 4 = 16
Check. Check with a subtraction: 16 − 4 = 12. ✓
Watch out. Write the sign, then the total after the equals sign.
Method. Use take away for fewer. Write the start, the sign, the number taken away and the answer.
- Write the start. There are 10 bees.10
- Write the sign. Fly away means take away, so use a minus sign.10 −
- Write the rest and the answer. 7 fly away.10 − 7 = 3
- Check. 10 − 7 = 3
Check. Check with addition: 3 + 7 = 10. ✓
Watch out. The minus sign is for taking away. The equals sign shows the answer.
Method. Use add for more. Write the start, the sign, the number added and the answer.
- Write the start. Mei starts with 14.14
- Write the sign. More means add, so use a plus sign.14 +
- Write the rest and the answer. 2 more are found.14 + 2 = 16
- Check. 14 + 2 = 16
Check. Check with a subtraction: 16 − 2 = 14. ✓
Watch out. Write the sign, then the total after the equals sign.
Method. Use take away for fewer. Write the start, the sign, the number taken away and the answer.
- Write the start. There are 16 bees.16
- Write the sign. Fly away means take away, so use a minus sign.16 −
- Write the rest and the answer. 14 fly away.16 − 14 = 2
- Check. 16 − 14 = 2
Check. Check with addition: 2 + 14 = 16. ✓
Watch out. The minus sign is for taking away. The equals sign shows the answer.
Method. Use the inverse: take away one of the parts from the answer. Do you get the other part?
- Write the sum. 9 + 7 = 15.9 + 7 = 15
- Do the inverse. Take 7 away from 15.15 - 7 = 8
- Compare with the start. We started with 9.8 is not 9
- Write the answer. No
Check. The correct answer is 16. ✓
Watch out. If the inverse does not give you the start, the sum is wrong.
Method. Use the inverse: add back the number taken away. Do you get the start?
- Write the subtraction. 17 − 8 = 9.17 − 8 = 9
- Do the inverse. Add 8 to 9.9 + 8 = 17
- Compare with the start. We started with 17.17 is the same as 17
- Write the answer. Yes
Check. 9 is the answer, so the sum is right. ✓
Watch out. Add back to check a subtraction.
Method. Use the inverse: take away one of the parts from the answer. Do you get the other part?
- Write the sum. 9 + 4 = 13.9 + 4 = 13
- Do the inverse. Take 4 away from 13.13 - 4 = 9
- Compare with the start. We started with 9.9 is the same as 9
- Write the answer. Yes
Check. 13 is the answer, so the sum is right. ✓
Watch out. If the inverse does not give you the start, the sum is wrong.
Method. Use the inverse: add back the number taken away. Do you get the start?
- Write the subtraction. 16 − 5 = 11.16 − 5 = 11
- Do the inverse. Add 5 to 11.11 + 5 = 16
- Compare with the start. We started with 16.16 is the same as 16
- Write the answer. Yes
Check. 11 is the answer, so the sum is right. ✓
Watch out. Add back to check a subtraction.
Method. Use the inverse: take away one of the parts from the answer. Do you get the other part?
- Write the sum. 7 + 4 = 11.7 + 4 = 11
- Do the inverse. Take 4 away from 11.11 - 4 = 7
- Compare with the start. We started with 7.7 is the same as 7
- Write the answer. Yes
Check. 11 is the answer, so the sum is right. ✓
Watch out. If the inverse does not give you the start, the sum is wrong.
Method. Use the inverse: add back the number taken away. Do you get the start?
- Write the subtraction. 13 − 6 = 7.13 − 6 = 7
- Do the inverse. Add 6 to 7.7 + 6 = 13
- Compare with the start. We started with 13.13 is the same as 13
- Write the answer. Yes
Check. 7 is the answer, so the sum is right. ✓
Watch out. Add back to check a subtraction.
Method. Use the inverse: take away one of the parts from the answer. Do you get the other part?
- Write the sum. 13 + 7 = 20.13 + 7 = 20
- Do the inverse. Take 7 away from 20.20 - 7 = 13
- Compare with the start. We started with 13.13 is the same as 13
- Write the answer. Yes
Check. 20 is the answer, so the sum is right. ✓
Watch out. If the inverse does not give you the start, the sum is wrong.
Method. Use the inverse: add back the number taken away. Do you get the start?
- Write the subtraction. 14 − 9 = 6.14 − 9 = 6
- Do the inverse. Add 9 to 6.6 + 9 = 15
- Compare with the start. We started with 14.15 is not 14
- Write the answer. No
Check. The correct answer is 5. ✓
Watch out. Add back to check a subtraction.