Estimate by rounding
3,482 + 1,191 is about 3,500 + 1,200, which is about 4,700.
The estimate does not need to be exact.
Year 4
Estimate and use inverse operations to check answers to a calculation
Use rounding to estimate and then use inverse operations to check whether an answer is reasonable.
Lesson overview
Number - addition and subtraction
Estimating gives a quick idea of the size of an answer. Inverse operations help us check exactly. Subtraction checks addition, and addition checks subtraction.
Use subtraction to check addition
If 2,643 + 1,205 = 3,848, then 3,848 - 1,205 should equal 2,643.
The inverse should undo the addition.
Use addition to check subtraction
If 5,000 - 1,275 = 3,725, then 3,725 + 1,275 should equal 5,000.
The check takes you back to the starting number.
Worked example
Check whether 4,281 + 618 = 4,899 is reasonable.
- Estimate the answer first by rounding 4,281 to 4,300 and 618 to 600.
- 4,300 + 600 is about 4,900.
- 4,899 is close to 4,900, so the answer sounds reasonable.
- Check exactly with subtraction: 4,899 - 618 = 4,281.
Round 1: Learn it
Estimate first, then choose the inverse operation that undoes the calculation.
Finish this round to unlock Round 2: Show you know it.
Question 1
What is a sensible estimate for 1,982 + 2,104?
Question 2
Which inverse check matches 3,245 + 760 = 4,005?
Question 3
Which estimate is best for 5,612 - 2,291?
Question 4
Use the inverse to check: if 6,000 - 1,450 = 4,550, then 4,550 + 1,450 =
Question 5
Why do we estimate first?
quick sensible guess
careful full calculation
Common mistakes
Watch for these as you work through the lesson.
- Treating the estimate as the exact answer.
- Choosing the wrong inverse operation.
- Rounding in a way that makes the estimate less helpful.
Related topics
These ideas connect closely to this lesson.
- Add and subtract numbers with up to 4 digits using the formal written methods of columnar addition and subtraction where appropriate.
- Solve addition and subtraction two-step problems in contexts, deciding which operations and methods to use and why.