Learn with Amar
Teaching video
A teaching recording for this chapter has not been published yet. Start with the worked example below and explore the related animations where available.
Grade 4 chapters and video availability01 · Read and understand
What you will learn
- Use division for a missing dimension and then calculate perimeter.
- Explain your method and check what the result means in the stated situation.
Before you start
Multiplication and division facts, simple fractions, place value, and basic length and area measurement.
Keep paper nearby. Read the question once for the context, then again to identify what is known and what you need to find.
Start with a question
A rectangular garden model has area 48 m² and length 8 m. Find its width and perimeter.
Why this math matters
Use a known area and one dimension to complete a rectangular layout. The worked example shows how to connect the situation, its mathematical representation, and a check on the result.
Set up the model
A useful answer starts with clear assumptions:
- Use the units, grouping rules, and reference whole stated in the question.
- Treat the supplied counts and measurements as exact within the classroom model, unless an estimate or a random outcome is requested.
02 · Work through the example
Follow the reasoning, one step at a time.
Try to predict the next step before reading it. After each calculation, explain why the operation makes sense and how it helps answer the original question.
Work through it with Amar
See this example unfold.
The complete worked example, one idea at a time.
Recover a rectangle from area and one side
PausedQuestion: Start with the question. Paused.
Question
Start with the question
A rectangular garden model has area 48 m² and length 8 m. Find its width and perimeter.
Before you calculate
Read what is known and what you need to find. Make a prediction before moving to the first calculation.
Starts paused. Play advances through the full text at a reading pace; pause whenever you need more time. Previous, Next, and the phase buttons let you set your own pace. Playback pauses when this walkthrough leaves the screen or you switch tabs.
Your device’s reduced-motion setting keeps each phase still. Manual controls remain available. The full written solution stays below.
Represent the situation
8 × width = 48
Area connects the two perpendicular side lengths.
Connect the parts
Width = 48 ÷ 8 = 6 m
Dividing square metres by metres leaves metres.
State and check the result
Perimeter = 2(8 + 6) = 28 m
Now all four boundary lengths are known.
The result
Perimeter = 2(8 + 6) = 28 m
Now all four boundary lengths are known.
Common mistakes to catch
- The given side is not the entire perimeter.
- Area uses square units, while recovered dimensions use length units.
03 · Practice independently
Try it before revealing the answer.
Use paper or a calculator as needed. Write your units and reasoning, then open the hint or explanation to check your approach.
Practice 1
A rectangle has area 35 cm² and width 5 cm. Find its length.
Show a hint
Divide area by width.
Reveal answer and explanation
7 cm
35 ÷ 5 = 7.
Practice 2
Can a rectangle's area alone determine its perimeter?
Show a hint
Compare different factor pairs of the same area.
Reveal answer and explanation
No
Area twelve can come from 2×6 or 3×4, with perimeters sixteen and fourteen.
Take the idea with you
Use a known area and one dimension to complete a rectangular layout.
04 · Reflect and continue
Can you explain it in your own words?
Before moving on, explain the main idea without looking at the worked example. Try both practice questions, check your reasoning, and name one mistake you now know how to avoid. Return to a step if you still need support.
Completion is your own study record, not a test score. It stays in this browser, does not sync to another device, and can be removed by clearing browser data.