It’s always a good idea to think about how we work out answers in our heads and doubling multiples of 5 is a good example of this. Thinking and saying what you do is similar to watching a slow motion movie, in real time everything happens very quickly but by slowing it down each step can be seen in turn.
Let’s look at doubling 65. I know without thinking that double 5 is 10, so I ignore this part and first work out double 60. This is 120. I then add on 10 for the double 5 part, making 130.
A similar process can be used with halving a number such as 130. I take 10 off to make 120 as it is easy to work out half of 120, which is 60. I then add on 5 (which is half of the 10 I took off at the start) making 65.
Try to get children to explain how they do these doubling and halving questions.
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