Primary 5 Foundation maths · Measurement & time · About 12 minutes
Find missing dimensions
Identify the surface to cover and the lengths that determine its rows. Keep missing or overlapping pieces visible.
Start the interactive lesson →Open practice activity
What you will learn
- Find missing dimensions
Models used: number sentences, stories, tables.
Before you start
How many squares in each row?
A rectangle covers 24 square units. It has four equal rows.
All the small boxes represent equally sized units, even when they belong to different rows.
A rectangle covers 24 square units. It has four equal rows.
One unit must have the same value in every row. That lets us compare amounts by counting their units.
Rows4 equal units · 4 units = 24
4 rows run across the picture; each row contains 6 objects.
Share the 24 squares equally between those four rows. There are six in each row.
24 ÷ 4 = 6
4 rows of 6 = 4 × 6 = 24
Each little square is one square unit. There are 6 squares across and 4 rows.
Six squares in each row means the missing length is 6 units.
6 × 4 = 24
What did the picture help us see?
Six squares in each row means the missing length is 6 units.
Worked example: A new example, together
A rectangle has perimeter 22 cm and width 6 cm. Find its length.
| Given measure | Known dimension |
|---|---|
| 22 | 6 cm |
Recover a missing dimension. Read each value with the heading above its column.
Identify the surface to cover and the lengths that determine its rows. Keep missing or overlapping pieces visible.
The same area or perimeter relationship can be used in reverse.
Recover a missing dimension Given measure Known dimension 22 6 cm Recover a missing dimension. Read each value with the heading above its column.
Half the perimeter is length + width = 11.
Recover a missing dimension Given measure Known dimension 22 6 cm A rectangle has perimeter 22 cm and width 6 cm. Find its length.
5 cm
Recover a missing dimension. Read each value with the heading above its column.
Subtract the width: 11 − 6.
Recover a missing dimension Given measure Known dimension 22 6 cm Recover a missing dimension. Read each value with the heading above its column.
Length = 5 cm.
Recover a missing dimension Given measure Known dimension 22 6 cm Recover a missing dimension. Read each value with the heading above its column.
Check the result against the question we started with.
2 × (5 + 6) = 22.
We found 5 cm. The same area or perimeter relationship can be used in reverse.
Recover a missing dimension Given measure Known dimension 22 6 cm
Common mistakes to discuss
- Halving the perimeter gives the length. Half the perimeter is one length plus one width. With perimeter 22 cm and width 6 cm, the length is 11 − 6 = 5 cm.
Check your understanding
- Find missing dimensions
- explain area calculations using covered square units
- check the answer against the original quantities and units
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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