Year 5

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We can estimate and check the result of a calculation
We can use inverse operations to estimate and check calculations

Year 5 Unit 9

What we are learning:

  • It is important that before carrying out any calculation, children have an idea of the approximate size of the answer so that they can tell whether the actual answer they calculate is of the correct size. We can do this by rounding the numbers in the calculation to the nearest 1, 10, 100 or 1000, etc to make them easier to calculate. For example, if we want to calculate 23 x 29, we might say that 23 is close to 20 and 29 is close to 30 and then 20 x 30 = 600. So we know the answer will be near to 600.
  • Children also need to realise that once they have done the calculation, they can compare it to the approximation as a way of checking.
  • Another way of checking the result of a calculation is to use the inverse operation. The inverse operation is the operation that ‘undoes’ the calculation or gets you back to the number you started with.
  • For example, if the correct answer to the calculation 78 + 135 is 205, then subtracting 135 from 205 should leave 78.
  • The inverse of addition is subtraction and vice versa.
  • The inverse of multiplication is division and vice versa.

Activities you can do at home:

Give your child a selection of calculations and ask them to approximate the answer and then calculate the answer. If the focus is approximation they can calculate the actual answer with a calculator. Ask them to compare their answer with the approximation. If there is a large difference then it would be a good idea for them to calculate the answer again and to consider their estimation strategy.

Examples are:
56 x 12, 72 x 15, 32 x 21, 564 + 148, 785 + 294, 521 – 117, 789 – 354, 204 ÷ 12, 135 ÷ 9

Good questions to ask:

Are you going to round this number to the nearest 10, 100 or 1000?
Roughly, what will the answer to this calculation be?
How do you know that this calculation is probably right?
Could you check it a different way?

If your child:

Is reluctant to round and estimate and wants to go straight to the calculation to ‘save time’
Talk together about the value of estimating – sometimes we don’t need a precise answer, and it’s a great way to check we are right!

Extension Activity

Please use this activity when you think your child understands the unit of work. It will deepen and extend your child’s understanding of this unit.


Extension activity (PDF)

Extension activity answers (PDF)