Primary 1 maths · Operations · About 10 minutes
Missing numbers in take-away
Find the starting number when the parts are known
Start the interactive lesson →Open practice activity
What you will learn
- Find the starting number when the parts are known
- Find how many were taken away
- Link each subtraction to a matching addition
Models used: bar models, number bonds, number lines, number sentences, stories.
Before you start
Ask: is the box the whole or a part?
We have 5 counters and take away 2. How many are left? (5 − 2)
Each counter stands for one object. Count each counter once to find how many there are.
Start with 5 counters.
The picture starts with 5 counters. Crosses mark 2 taken away. Count only the counters without crosses to find what remains.
Take away 2. Cross them out.
The picture starts with 5 counters. Crosses mark 2 taken away. Count only the counters without crosses to find what remains.
Count what is left: 1, 2, 3.
The picture starts with 5 counters. Crosses mark 2 taken away. Count only the counters without crosses to find what remains.
So 5 − 2 = 3.
5 − 2 = 3
Blue counters stayed. Orange counters are the 2 we put back. Together they restore our starting 5.
Put back what we took away. Check: 3 + 2 = 5.
3 + 2 = 5
32
Why does adding find the starting number?
Taking away split the whole into two parts: the part taken and the part left. Putting the two parts back together rebuilds the whole. That is why 7 − 4 = 3 and 4 + 3 = 7 describe the same bond.
Worked example
8 − □ = 5
What does the box stand for?
In 8 − □ = 5, the box is…
the part taken away
8 is the whole. 5 is the part left.
The part taken away is the whole take away the part left.
8 − 5 = ?
What is 8 − 5?
3
So 3 were taken away.
□ = 3
Check it.
8 − 3 = 5 ✓
Common mistakes to discuss
- Subtracting whatever numbers are shown. Ask what the box stands for. In □ − 4 = 3 the box is the whole we started with, so we put the parts together: 4 + 3.
- Thinking subtraction only means “take away”. Subtraction also finds a missing part and a difference. The bar model shows which one we need.
Check your understanding
- solve □ − 4 = 3 and 8 − □ = 5
- tell whether the box is the whole or a part
- show a take-away story with a bar model
- check a subtraction with an addition
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.
Try this lesson →Related Primary 1 lessons
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