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Cakes and Calculations

Cakes and Calculations. Subtraction. Mental Strategies for Subtraction. Secure mental subtraction requires the ability to: recall key subtraction facts instantly (inverse of number pairs to 10, 20 & 100, halves etc) and to apply these to similar calculations

sigourney-wiley
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Cakes and Calculations

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  1. Cakes and Calculations Subtraction

  2. Mental Strategies for Subtraction Secure mental subtraction requires the ability to: • recall key subtraction facts instantly (inverse of number pairs to 10, 20 & 100, halves etc) and to apply these to similar calculations • mentally subtract combinations of one and two digit numbers • understand that subtraction is the inverse of addition and recognise that subtraction can’t be done in any order (it has to start with the larger number) • understand the language of subtraction including less, minus, take away, difference between etc)

  3. Written methods for Subtraction Stage 1: The empty number line The empty number line helps to record the steps in mental subtraction. There are several ways to do this: • Counting Back - a calculation like 74 - 27 can be recorded by counting back 27 from 74 to reach 47. or • Counting Up - the steps can also be recorded by counting up from the smaller number to find the difference or

  4. Written methods for Subtraction Stage 1: The empty number line With practice, children will need to record less information and decide whether to count back or forward. It is useful to ask children whether counting up or back is the more efficient for calculations such as 57 - 12, 86 - 77 or 43 - 28.

  5. Over to you! Use a number line to find answers to these sums: 59 - 13 (count up or back?) 86 – 68 (count up or back?) 142 - 35

  6. Written methods for Subtraction Stage 2: Partitioning Subtraction can be recorded using partitioning to write equivalent calculations that are easier to carry out mentally. For 74 - 27 this involves partitioning the 27 into 20 and 7, then subtracting 20 and 7 in turn. 74 – 27 is the same as 74 – 20 – 7 74 – 20 = 54 54 – 7 = 47

  7. Written methods for Subtraction Stage 2: Partitioning When subtracting, it is easier not to partition both numbers as children will commonly reverse the unit digits if they can’t get an answer the right way round! 74 – 27 70 – 20 = 50 4 – 7 = 3 50 + 3 = 53

  8. Over to you! Use partitioning to find these answers: 65 - 38 94 – 45 108 - 53

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