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We can estimate and check calculations by approximating and using inverse operations
Year 6 Unit 8a
What we are learning:
- Estimation s a key skill in maths and in everyday life. Sometimes we do not need to know a precise answer, but we do need to know an approximate answer to check whether something is correct.
- For example, if we bought 4 items for £2.70, £13.20, 85p and £6.99 we might need to know roughly how much the total is to hand over sufficient money. If we round these amounts to the nearest pound we would have
£3 + £13 + £1 + £7 which can be mentally added to get £24 as an estimate of the total.
- There is no need to round these numbers and write them down – with practice it can become a mental process to get an approximate answer
- We can check calculations by repeating them or by using a calculator.
- We can also check answers by using inverse operations. If we are asked to add £3.57 and £2.36 we can calculate the answer to be £5.93. Since subtraction is the inverse operation to addition we can check the calculation by taking our answer of £5.93 and subtracting £2.36. If our answer is right we should get back to £3.57.
Activities you can do at home:
Have a look at the Activity Sheet to estimate the answers to the calculations. Check them by working them out accurately on paper or by using a calculator to see how near you were.
Good questions to ask:
What is the approximate answer to this?
How will you round these numbers?
What’s your best estimate?
How near were you?
How might you have got nearer to the correct answer without working anything out in detail?
If your child:
Tries to work out the precise answer rather than estimating
Encourage them to work at speed and not to focus on the detail, or encourage them to round the numbers in their head
Is disappointed that their estimate is not exactly the same as their checked calculation
Remind them that estimates are not meant to be precisely accurate – they are a near guess, so the question to explore is How near was your estimate?