Step-by-step Maths Level 2 Adding and subtracting within 100: Adding a 1-digit number to a 2-digit number
Part of the Step-by-step Maths Level 2 adding and subtracting within 100 chapter. Chapter overview · whole chapter on one page
Adding a 1-digit number to a 2-digit number
Split the 2-digit number into tens and ones. Add the 1-digit number to the ones.
If the ones still add up to 9 or less, the tens do not change. Put the tens back.
Key ideas
- Split the number into tens and ones.
- Add to the ones.
- Put the tens back on the total.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 34 into 30 and 4. The tens and the ones are kept apart.34 = 30 + 4
- Add the ones: 4 + 5. 4 + 5 = 9 is less than 10, so no tens change.4 + 5 = 9
- Put the tens back: 30 + 9. Add the tens and the ones together.30 + 9 = 39
- Write the answer. 39
Check. Count on 5 from 34: you reach 39.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 61 into 60 and 1. The tens and the ones are kept apart.61 = 60 + 1
- Add the ones: 1 + 3. 1 + 3 = 4 is less than 10, so no tens change.1 + 3 = 4
- Put the tens back: 60 + 4. Add the tens and the ones together.60 + 4 = 64
- Write the answer. 64
Check. Count on 3 from 61: you reach 64.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 27 into 20 and 7. The tens and the ones are kept apart.27 = 20 + 7
- Add the ones: 7 + 2. 7 + 2 = 9 is less than 10, so no tens change.7 + 2 = 9
- Put the tens back: 20 + 9. Add the tens and the ones together.20 + 9 = 29
- Write the answer. 29
Check. Count on 2 from 27: you reach 29.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Watch out
Watch out. Keep the tens. 34 + 5 is 39, not 310 and not 49.
Exam tip
Grown-up tip. Make the number with tens sticks and ones cubes. Add the cubes to the ones pile and count.
Exercise 7A Practice: adding a 1-digit number
Non-calculatorMethod. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 50 into 50 and 0. The tens and the ones are kept apart.50 = 50 + 0
- Add the ones: 0 + 8. 0 + 8 = 8 is less than 10, so no tens change.0 + 8 = 8
- Put the tens back: 50 + 8. Add the tens and the ones together.50 + 8 = 58
- Write the answer. 58
Check. Count on 8 from 50: you reach 58.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 60 into 60 and 0. The tens and the ones are kept apart.60 = 60 + 0
- Add the ones: 0 + 8. 0 + 8 = 8 is less than 10, so no tens change.0 + 8 = 8
- Put the tens back: 60 + 8. Add the tens and the ones together.60 + 8 = 68
- Write the answer. 68
Check. Count on 8 from 60: you reach 68.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 78 into 70 and 8. The tens and the ones are kept apart.78 = 70 + 8
- Add the ones: 8 + 1. 8 + 1 = 9 is less than 10, so no tens change.8 + 1 = 9
- Put the tens back: 70 + 9. Add the tens and the ones together.70 + 9 = 79
- Write the answer. 79
Check. Count on 1 from 78: you reach 79.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 85 into 80 and 5. The tens and the ones are kept apart.85 = 80 + 5
- Add the ones: 5 + 3. 5 + 3 = 8 is less than 10, so no tens change.5 + 3 = 8
- Put the tens back: 80 + 8. Add the tens and the ones together.80 + 8 = 88
- Write the answer. 88
Check. Count on 3 from 85: you reach 88.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 30 into 30 and 0. The tens and the ones are kept apart.30 = 30 + 0
- Add the ones: 0 + 8. 0 + 8 = 8 is less than 10, so no tens change.0 + 8 = 8
- Put the tens back: 30 + 8. Add the tens and the ones together.30 + 8 = 38
- Write the answer. 38
Check. Count on 8 from 30: you reach 38.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 13 into 10 and 3. The tens and the ones are kept apart.13 = 10 + 3
- Add the ones: 3 + 6. 3 + 6 = 9 is less than 10, so no tens change.3 + 6 = 9
- Put the tens back: 10 + 9. Add the tens and the ones together.10 + 9 = 19
- Write the answer. 19
Check. Count on 6 from 13: you reach 19.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 10 into 10 and 0. The tens and the ones are kept apart.10 = 10 + 0
- Add the ones: 0 + 1. 0 + 1 = 1 is less than 10, so no tens change.0 + 1 = 1
- Put the tens back: 10 + 1. Add the tens and the ones together.10 + 1 = 11
- Write the answer. 11
Check. Count on 1 from 10: you reach 11.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 46 into 40 and 6. The tens and the ones are kept apart.46 = 40 + 6
- Add the ones: 6 + 3. 6 + 3 = 9 is less than 10, so no tens change.6 + 3 = 9
- Put the tens back: 40 + 9. Add the tens and the ones together.40 + 9 = 49
- Write the answer. 49
Check. Count on 3 from 46: you reach 49.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 34 into 30 and 4. The tens and the ones are kept apart.34 = 30 + 4
- Add the ones: 4 + 1. 4 + 1 = 5 is less than 10, so no tens change.4 + 1 = 5
- Put the tens back: 30 + 5. Add the tens and the ones together.30 + 5 = 35
- Write the answer. 35
Check. Count on 1 from 34: you reach 35.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 50 into 50 and 0. The tens and the ones are kept apart.50 = 50 + 0
- Add the ones: 0 + 2. 0 + 2 = 2 is less than 10, so no tens change.0 + 2 = 2
- Put the tens back: 50 + 2. Add the tens and the ones together.50 + 2 = 52
- Write the answer. 52
Check. Count on 2 from 50: you reach 52.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 72 into 70 and 2. The tens and the ones are kept apart.72 = 70 + 2
- Add the ones: 2 + 4. 2 + 4 = 6 is less than 10, so no tens change.2 + 4 = 6
- Put the tens back: 70 + 6. Add the tens and the ones together.70 + 6 = 76
- Write the answer. 76
Check. Count on 4 from 72: you reach 76.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 73 into 70 and 3. The tens and the ones are kept apart.73 = 70 + 3
- Add the ones: 3 + 3. 3 + 3 = 6 is less than 10, so no tens change.3 + 3 = 6
- Put the tens back: 70 + 6. Add the tens and the ones together.70 + 6 = 76
- Write the answer. 76
Check. Count on 3 from 73: you reach 76.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 80 into 80 and 0. The tens and the ones are kept apart.80 = 80 + 0
- Add the ones: 0 + 7. 0 + 7 = 7 is less than 10, so no tens change.0 + 7 = 7
- Put the tens back: 80 + 7. Add the tens and the ones together.80 + 7 = 87
- Write the answer. 87
Check. Count on 7 from 80: you reach 87.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 53 into 50 and 3. The tens and the ones are kept apart.53 = 50 + 3
- Add the ones: 3 + 5. 3 + 5 = 8 is less than 10, so no tens change.3 + 5 = 8
- Put the tens back: 50 + 8. Add the tens and the ones together.50 + 8 = 58
- Write the answer. 58
Check. Count on 5 from 53: you reach 58.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 72 into 70 and 2. The tens and the ones are kept apart.72 = 70 + 2
- Add the ones: 2 + 5. 2 + 5 = 7 is less than 10, so no tens change.2 + 5 = 7
- Put the tens back: 70 + 7. Add the tens and the ones together.70 + 7 = 77
- Write the answer. 77
Check. Count on 5 from 72: you reach 77.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.
Method. Split the 2-digit number into tens and ones. Add the ones, then put the tens back.
- Split 75 into 70 and 5. The tens and the ones are kept apart.75 = 70 + 5
- Add the ones: 5 + 4. 5 + 4 = 9 is less than 10, so no tens change.5 + 4 = 9
- Put the tens back: 70 + 9. Add the tens and the ones together.70 + 9 = 79
- Write the answer. 79
Check. Count on 4 from 75: you reach 79.
Watch out. Only the ones change when you add a 1-digit number and the ones do not go past 9.