Estimate by rounding
248 + 149 is about 250 + 150, which is about 400.
An estimate does not have to be exact. It helps us judge the size of the answer.
Year 3
Estimate the answer to a calculation and use inverse operations to check answers
Estimate sensible answers and use inverse operations to check calculations.
Lesson overview
Number - addition and subtraction
Estimating helps us decide whether an answer is reasonable. We can round numbers to make a quick guess. We can also use the inverse operation to check, such as subtracting to check an addition.
Use subtraction to check addition
If 326 + 143 = 469, then 469 - 143 should equal 326.
Inverse operations undo each other.
Use addition to check subtraction
If 500 - 178 = 322, then 322 + 178 should equal 500.
The check should take us back to the starting number.
Worked example
Check whether 287 + 201 = 488 is reasonable.
- Estimate the answer first by rounding 287 to 300 and 201 to 200.
- 300 + 200 is about 500.
- 488 is close to 500, so it sounds reasonable.
- Use subtraction to check exactly: 488 - 201 = 287.
Try it
Estimate first, then think about which inverse check matches the calculation.
Question 1
What is a sensible estimate for 198 + 103?
Question 2
Which inverse check matches 345 + 120 = 465?
Question 3
Which estimate is best for 612 - 291?
Question 4
Use the inverse to check: if 520 - 180 = 340, then 340 + 180 =
Question 5
Why do we estimate first?
quick sensible guess
careful full calculation
Common mistakes
Watch for these when working through the lesson.
- Treating the estimate as the exact answer.
- Using the wrong inverse operation.
- Rounding both numbers in an unhelpful way.
Related topics
These ideas fit closely with this lesson.
- Add and subtract numbers mentally, including a three-digit number and ones, tens and hundreds.
- Solve problems, including missing number problems, using number facts, place value, and more complex addition and subtraction.