Primary 1 maths · Operations · About 10 minutes
Written addition within 100
Line up tens and ones, exchange ten ones for one ten, and complete written addition within 100
Start the interactive lesson →Practise: Number bridge to 100
What you will learn
- Line up tens and ones, exchange ten ones for one ten, and complete written addition within 100
Models used: place-value charts, number sentences, bar models, stories.
Before you start
Give each place its own column
Work out 28 + 35.
Line up ones under ones and tens under tens. Start on the right.
28 + 35
tens ones Carried units First number 2 8 + 3 5 Answer ? ? 13 ones make 1 ten and 3 ones. Write 3 here. Carry 1 to the next column.
8 + 5 = 13
tens ones Carried units 1 First number 2 8 + 3 5 Answer ? 3 1 ten⇄10 ones
Add the tens, including the carried 1. Write 6 in this column.
2 + 3 + 1 = 6
tens ones Carried units 1 First number 2 8 + 3 5 Answer 6 3 Read the answer from left to right. Subtract one addend to check.
63 − 35 = 28
tens ones Carried units 1 First number 2 8 + 3 5 Answer 6 3
Worked example: Fill one answer box at a time
Work out 46 + 27.
| tens | ones | |
|---|---|---|
| Carried units | ||
| First number | 4 | 6 |
| + | 2 | 7 |
| Answer | ? | ? |
Line up ones under ones and tens under tens. Start on the right.
46 + 27
tens ones Carried units First number 4 6 + 2 7 Answer ? ? Make the exchange. Then fill the highlighted answer box.
tens ones Carried units 1 First number 4 6 + 2 7 Answer ? ? 1 ten⇄10 ones
What digit do we write in the ones column?
3
Work in the highlighted column.
tens ones Carried units 1 First number 4 6 + 2 7 Answer ? 3 What digit belongs in the tens column?
7
Read the answer from left to right. Subtract one addend to check.
73 − 27 = 46
tens ones Carried units 1 First number 4 6 + 2 7 Answer 7 3
Common mistakes to discuss
- Forget a carried unit. Include the small carried digit above the next column.
- Write two digits in one box. Keep one digit in each answer column; exchange ten units for one next-place unit.
Check your understanding
- Line up matching places.
- Exchange ten ones for a ten and include every carried unit.
- Check the total with subtraction.
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
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