Primary 4 maths · Whole numbers · About 10 minutes
Estimate and check calculations
Locate both endpoints, then the halfway point and the number itself.
Start the interactive lesson →Related practice: Hundred-thousand tower
What you will learn
- Estimate and check calculations
Models used: number sentences, stories, diagrams.
Before you start
Which ten is closer?
46 sits between 40 and 50. Which end is closer?
Start at the marked number. A jump to the right adds; a jump to the left takes away. Read the size of each jump.
46 sits between 40 and 50. Which end is closer?
Start at the marked number. A jump to the right adds; a jump to the left takes away. Read the size of each jump.
45 is halfway. 46 has already passed the halfway point towards 50.
Start at the marked number. A jump to the right adds; a jump to the left takes away. Read the size of each jump.
It takes 4 steps to reach 50, but 6 to reach 40. So 46 rounds to 50.
46 ≈ 50
The complete bar is the whole. Each labelled section is one part of it.
An estimate checks the size of an answer: 46 + 32 is about 50 + 30, or 80.
46 + 32 = 78, close to 80
What did the picture help us see?
An estimate checks the size of an answer: 46 + 32 is about 50 + 30, or 80.
Worked example: A new example, together
Round 80 to the nearest 10.
Locate both endpoints, then the halfway point and the number itself.
Rounding chooses a nearby benchmark, so the result is approximate.
808090The number lies between neighbouring multiples. Neighbouring multiples: 80 and 90.
808090The number lies between neighbouring multiples. Round 80 to the nearest 10.
80
The halfway point is 85.
808090The number lies between neighbouring multiples. Choose the nearer multiple; a halfway value rounds up: 80.
808090The number lies between neighbouring multiples. Check the result against the question we started with.
80 ≈ 80; the distance is no more than 5.
We found 80. Rounding chooses a nearby benchmark, so the result is approximate.
808090The number lies between neighbouring multiples.
Common mistakes to discuss
- Reading every following digit. Only the digit in the next smaller place decides. Rounding 2449 to the nearest hundred looks at the 4 tens, so it gives 2400, not 2500.
Check your understanding
- Estimate and check calculations
- use neighbouring benchmarks and the midpoint to justify an estimate
- 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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