Estimate first
48,210 + 31,790 is about 48,000 + 32,000.
A quick estimate tells us the answer should be near 80,000.
Year 5
Use rounding to check answers to calculations and determine, in the context of a problem, levels of accuracy
Use estimation and inverse operations to check whether answers make sense.
Lesson overview
Number - addition and subtraction
Before or after a calculation, we can round the numbers to get a quick estimate. We can also use the inverse operation to check an answer. These checks help us spot mistakes and judge whether an answer is sensible in context.
Check with the inverse
If 63,540 - 18,200 = 45,340, then 45,340 + 18,200 should give 63,540.
The inverse operation checks the original answer.
Think about accuracy in context
If you are counting people at a stadium, an answer of 4 might not be sensible.
stadium crowd
a sensible scale
The context tells us what range of answer is reasonable.
Worked example
Check whether 27,814 + 15,291 = 52,105 is sensible.
- Round 27,814 to about 28,000 and 15,291 to about 15,000.
- The estimate is about 43,000.
- 52,105 is too far from the estimate.
- So the answer is not sensible.
Round 1: Learn it
Estimate first, then decide whether the exact answer fits.
Finish this round to unlock Round 2: Show you know it.
Question 1
About what is 48,210 + 31,790?
Question 2
If 63,540 - 18,200 = 45,340, which inverse check is correct?
Question 3
Is 27,814 + 15,291 = 52,105 sensible?
Question 4
About what is 91,200 - 39,600?
Question 5
A class has 31 children. Is an answer of 310 sensible for the class size?
one class
Common mistakes
Watch for these as you work through the lesson.
- Using an estimate that is too rough to be helpful.
- Checking with the wrong inverse operation.
- Ignoring the context when deciding whether an answer is sensible.
Related topics
These ideas connect closely to this lesson.
- Add and subtract whole numbers with more than 4 digits, including using formal written methods where appropriate.
- Solve addition and subtraction multi-step problems in contexts, deciding which operations and methods to use and why.