Year 1: Missing numbers and one-step problems
Part of the Year 1 adding and subtracting within 20 chapter. Chapter overview · whole chapter on one page · Download worksheet (PDF) · Answers (PDF)
Missing numbers and one-step problems
A missing number is a number we do not know yet. We show it with a box, like this: 6 + ? = 15. To find it, we do the opposite. Adding undoes taking away, and taking away undoes adding.
A one-step problem is a story with just one add or one take-away. Read the story, then decide: do we add or do we take away?
Key ideas
- A box or a question mark shows the missing number.
- To find a missing number, do the opposite.
- In a story, “more” or “altogether” means add.
- In a story, “gone” or “left” means take away.
Key words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 15
- Take the number you know away from the total. Taking away undoes adding.15 − 6 = 9
- Write the missing number. 9
Check. Check: 6 + 9 = 15. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Method. Think about how many were taken away. Take what is left away from the start.
- Start with 14. This is the number we begin with.Start: 14
- Find what is left: 9. The answer to the take-away is what is left.Left: 9
- Take what is left away from the start. The difference is how many were taken away.14 − 9 = 5
- Write the missing number. 5
Check. Check: 14 − 5 = 9. ✓
Watch out. The missing number is the amount taken away. It is not the amount left.
Method. Put back the number that was taken away. Adding undoes taking away.
- Look at the take-away. Something was taken away and the answer is what is left.? − 4 = 11
- Add back the number that was taken away. Adding is the opposite of taking away.11 + 4 = 15
- Write the missing number. 15
Check. Check: 15 − 4 = 11. ✓
Watch out. Adding is the opposite of taking away. Use it to find the number you started with.
Watch out
Watch out: for a missing number in a sum, do not add the two numbers you can see. Take the known number away from the total.
Exam tip
Grown-up tip. Use a sum with a box, like 5 + ? = 13. Ask: “What number makes 13 with 5?” Then check: add the number back on to make sure it is right.
Exercise 5F Missing numbers and one-step problems
Non-calculatorKey words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 13
- Take the number you know away from the total. Taking away undoes adding.13 − 5 = 8
- Write the missing number. 8
Check. Check: 5 + 8 = 13. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Key words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 16
- Take the number you know away from the total. Taking away undoes adding.16 − 9 = 7
- Write the missing number. 7
Check. Check: 9 + 7 = 16. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Method. Think about how many were taken away. Take what is left away from the start.
- Start with 17. This is the number we begin with.Start: 17
- Find what is left: 12. The answer to the take-away is what is left.Left: 12
- Take what is left away from the start. The difference is how many were taken away.17 − 12 = 5
- Write the missing number. 5
Check. Check: 17 − 5 = 12. ✓
Watch out. The missing number is the amount taken away. It is not the amount left.
Method. Think about how many were taken away. Take what is left away from the start.
- Start with 15. This is the number we begin with.Start: 15
- Find what is left: 6. The answer to the take-away is what is left.Left: 6
- Take what is left away from the start. The difference is how many were taken away.15 − 6 = 9
- Write the missing number. 9
Check. Check: 15 − 9 = 6. ✓
Watch out. The missing number is the amount taken away. It is not the amount left.
Method. Put back the number that was taken away. Adding undoes taking away.
- Look at the take-away. Something was taken away and the answer is what is left.? − 6 = 7
- Add back the number that was taken away. Adding is the opposite of taking away.7 + 6 = 13
- Write the missing number. 13
Check. Check: 13 − 6 = 7. ✓
Watch out. Adding is the opposite of taking away. Use it to find the number you started with.
Key words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 11
- Take the number you know away from the total. Taking away undoes adding.11 − 4 = 7
- Write the missing number. 7
Check. Check: 4 + 7 = 11. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Method. Think about how many were taken away. Take what is left away from the start.
- Start with 12. This is the number we begin with.Start: 12
- Find what is left: 3. The answer to the take-away is what is left.Left: 3
- Take what is left away from the start. The difference is how many were taken away.12 − 3 = 9
- Write the missing number. 9
Check. Check: 12 − 9 = 3. ✓
Watch out. The missing number is the amount taken away. It is not the amount left.
Method. Put back the number that was taken away. Adding undoes taking away.
- Look at the take-away. Something was taken away and the answer is what is left.? − 5 = 8
- Add back the number that was taken away. Adding is the opposite of taking away.8 + 5 = 13
- Write the missing number. 13
Check. Check: 13 − 5 = 8. ✓
Watch out. Adding is the opposite of taking away. Use it to find the number you started with.
Key words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 20
- Take the number you know away from the total. Taking away undoes adding.20 − 8 = 12
- Write the missing number. 12
Check. Check: 8 + 12 = 20. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Method. Think about how many were taken away. Take what is left away from the start.
- Start with 15. This is the number we begin with.Start: 15
- Find what is left: 9. The answer to the take-away is what is left.Left: 9
- Take what is left away from the start. The difference is how many were taken away.15 − 9 = 6
- Write the missing number. 6
Check. Check: 15 − 6 = 9. ✓
Watch out. The missing number is the amount taken away. It is not the amount left.
Key words. The box (?) shows the missing number.
Method. To find the missing number, take the number you know away from the total.
- Look at the total. The total is the answer to the sum.Total: 15
- Take the number you know away from the total. Taking away undoes adding.15 − 8 = 7
- Write the missing number. 7
Check. Check: 8 + 7 = 15. ✓
Watch out. Do not add the two numbers together. Take the number you know away from the total.
Key words. To count on is to keep going up from a number, one at a time.
Method. Start with the bigger number, 12. Then count on 5.
- Start at 12. Start with the bigger number. You do not need to start at 1.Start: 12
- Count on 5 more. Say each number as you move one step on.13, 14, 15, 16, 17
- Write the answer. The last number you say is the answer.17
Check. Count back 5 from 17. You reach 12 again. ✓
Watch out. Count on from 12. Do not say 12 again as the first step.
Method. A fact family uses the same three numbers, 8, 7 and 15.
- Look at the first fact. Priya adds 8 and 7 to make 15.8 + 7 = 15
- Look at the second fact. Priya takes 7 away from 15 to get 8. The same three numbers are used.15 − 7 = 8
- Write the answer. Yes. Both facts use 8, 7 and 15.
Check. Take 8 away from 15 to check: 15 − 8 = 7. ✓
Watch out. Facts in the same family use the same three numbers.