Finding the Missing Number

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1 Getting Started
2 Watch and Notice
3 Try It Together
4 Work It in Your Copy
5 Class Challenge
Pupil practice

1 - Getting Started ~4 mins

Look at the balance. On one pan there are 10 cubes. On the other pan there are 4 cubes and a covered box. The beam is level, so both pans weigh the same.

How many cubes are hiding under the box?

box + 4 = 10

Teacher Notes

Take three hands-up answers, not open call-outs. Give five seconds of quiet think-time before any hands go up. Do not reveal the answer yet — that is the job of the next step. The box starts empty on screen, so nothing is given away.

👇 Mark your steps as done as you complete them.

+5 XP

2 - Watch and Notice ~9 mins

Watch these balances. Each one has a box hiding a number, and the whole is on the other pan. Each balance shows the missing number that makes the beam level.

Watch this one, the whole number is now on the left, and the box is on the right.

This last one takes away. Here the box is the whole, so it must hold both parts. Look at the box pan holding both parts together, the box holds more than either part on its own.

Now on the take-away balance the box pan and the other pan both weigh the same. So how many does the box hold?

Teacher Notes

□ + 4 = 10 — ask for the missing part before revealing; the whole is 10, one part is 4, so the box is 6. Say take the part you know away from the whole.
□ + 3 = 8 — same move, box is 5.
7 = □ + 2 — the whole (7) is now on the left. Hold out for pupils spotting that it is still whole-take-away-part, so the box is 5.
□ − 2 = 6 — the trickiest. Show the box-building balance first (slot 3): the box pan is built from 6 and 2 loaded together, so the class SEES the box must hold both parts. Then show the take-away balance (slot 4) and ask the class to name the number the box holds — do not say it first. It is 8 (6 + 2). Point at both pans so the class sees why the box grows.

👇 Mark your steps as done as you complete them.

+10 XP

3 - Try It Together ~11 mins

Now we work through these together at the board, finding the number hiding in the box each time:

□ + 5 = 11
□ + 6 = 13
12 = □ + 4
□ − 3 = 5

Slide the unknown on each balance until the beam hangs level, then read the box.

Find the missing number together

Teacher Notes

Work these together at the board — a pupil slides the unknown on each balance while the class agrees or corrects out loud, before anyone confirms the answer.

For each equation, ask what do we know and what are we hunting for? Is the box a part or the whole? before anyone slides the unknown. On 12 = □ + 4 hold out for the class noticing the whole is on the left this time, so the box is 8. On □ − 3 = 5 pause: the box is the whole now, so it must hold both parts put back together (5 + 3) — point at the pan holding the box so the class sees why it gets bigger. Do not say the total before pupils try it. After each level beam, ask a pupil to read the true sentence aloud.

+15 XP

4 - Work It in Your Copy ~2 mins

COPYBOOK MOMENT

In your maths copy, solve this number sentence. Write the missing number in the box, then write the full true sentence underneath.

□ + 5 = 12

Teacher Notes

Walk the room, glance for pupils writing the whole true sentence (7 + 5 = 12) and not just the box number — this is whole-class copybook practice, not marking.

+10 XP

5 - Class Challenge ~8 mins

Your turn to practise — find each hidden number:

□ + 2 = 9
□ + 7 = 15
13 = □ + 5
□ − 3 = 8

Remember: when the box is the whole, it holds both parts put back together. Level the beam each time, then press Check.

Find the missing number

Teacher Notes

Keep this round brisk and independent — pupils take turns at the board, check each answer, and the class confirms before moving on rather than over-explaining.

Ask before each Check: is the box a part or the whole here? The first three have the box as a part (take away to find it). □ − 3 = 8 has the box as the whole, so it holds both parts put back together (8 + 3) — the last and trickiest. Let pupils reach the total themselves before you confirm it.

+15 XP

Pupil practice

Module 10 · Algebra: Patterns, Rules and Number Sentences Mixed

Lesson 112 · Finding the Missing Number

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This lesson is copyright of Coding Ireland 2017 - 2025. Unauthorised use, copying or distribution is not allowed.

Teacher Class Feed

1 - Getting Started ~4 mins

box + 4 = 10

2 - Watch and Notice ~9 mins

3 - Try It Together ~11 mins

Find the missing number together

4 - Work It in Your Copy ~2 mins

5 - Class Challenge ~8 mins

Find the missing number

Unlock the full learning experience

XP