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We can explain why we choose to work mentally, or use a written method or a calculator
Year 5 Unit 10a
What we are learning:
- When faced with any calculation, children should be encouraged to first ask themselves if they are able to calculate the answer mentally. If not they should use a written method. If they feel that they cannot do the
calculation using a written method they then may choose to use a
- We want children to look at the numbers involved in the calculation and decide for themselves how they will calculate the answer rather than always take the same approach out of habit – this will vary for different
calculations depending on the numbers involved.
- The decision to calculate mentally, written or using a calculator is an individual one and therefore two children (or adults!) when faced with the same calculation may decide to work out the answer using different methods.
- Using a calculator should not be seen as cheating, especially for calculations involving decimals. Children need to have an understanding of what to tell the calculator to do.
- Jottings and informal notes can be very useful to support mental calculations.
ACTIVITY 1: WHICH METHOD SHALL I USE?
Activities you can do at home:
Give your child some calculations and ask them how they would go about solving them – would they do them mentally, on paper or with a calculator?
Identify how you would do the following calculations and explain why.
Sort the questions in three sets – would you calculate mentally, using a written method or using a calculator? Then calculate and discuss whether the chosen method was the most efficient.
Write each question on a piece of paper then sort them into groups, mental, written or calculator.
- 10,000 – 10
- 10,000 – 275.82
- 25 x 19
- 25 x 16.7
- 5 % of 86
- 84 – 77
- Half of 378
- 248 – 99
- 248.3 – 97.8
- 76.8 x 12
At the same time, make the same decisions yourself. When you have both decided, each of you can explain to the other which method you chose and why. Although it is absolutely acceptable that there may be differences,
making comparisons may highlight strategies that may not have been considered, especially when explaining the mental strategies used. E.g. one player may have said they would need to use a written method for 18 x 25 but
the other player may they did it mentally by doing 18 x 100 = 1800 and then dividing by 4 (half and half again) to give 450.
Ask your child to create an example of a calculation they would do mentally, one they would do using a written method and one they would do using a calculator. You may want to give certain criteria – e.g., they must be
multiplication calculations, each of the calculations must include at least one decimal with 2 decimals places, etc.
Good questions to ask:
Why did you choose that strategy?
Which method is the most efficient?
If you tried them again would you calculate some of the questions using a different strategy?
If your child:
Suggests a different method to the one you would have chosen
They are not ‘wrong’. Ask them to explain why they chose that method, and whether there are any other methods they might have chosen (they might not have considered them all). Then compare their choices together.