Idea 9of 22Builds onLength × width gives the area
Work back from the area
If you know the area and one side, how do you find the other?
Today you'll discover
- how rows help you work back
- why dividing undoes multiplying
- how to check the answer
Think firstA rectangle covers 24 m². It is 8 m long.
A rectangle covers 24 m² and is 8 m long. How wide is it?
Decide first, then press Next. Think: how many rows of 8 make 24?
An 8 by 3 rectangle on a grid. Squares fill it in rows of 8, with row 1: 8, row 2: 8, row 3: 8 beside them, and the line 3 rows of 8: 8 × 3 = 24 m². The last step shows a 9 by 6 rectangle for a card of 54 cm².
See the whole ideathe big idea and the key word, in one place
The big idea
To find a missing side, divide the area by the side you know.
Working back
Area 24 m², length 8 m: the width is 24 ÷ 8 = 3 m.
- area24 m²
- length8 m
- width3 m
Easy to get wrongSubtracting the side from the area mixes squares with a length. Divide instead.
Key word
- unknown length
- a side you work out instead of measuring
Ready to use it?
- Find a missing side from the area and the other side.