Tag: doubling

Year 4 mental arithmetic: multiplying by 4

Knowing the 4 times table is very important but using it is not always the most efficient way of finding an answer when using just mental methods. here we have a worksheet for year 4 children which looks at an alternative approach when multiplying by 4. This uses knowledge of the 2 times table to double and double again.

With some numbers this could be a better way. For example;

150 x4 can be done easily by doubling 150 to get 300 and then doubling 300 to get 600 – all done ‘in your head’ in less than a second!

Why not try this worksheet, to be found in the Year 4 calculating category.

Multiply by 4 by doubling

Year 3 maths worksheet: Double multiples of 5

There are a couple of techniques which help with being able to double multiples of 5 quickly ‘in your head’.

Firstly any 2-digit multiple of 5 which is also a multiple of ten (eg 60) is straightforward as only the tens digit needs to be considered.

It is harder to double numbers such as 65. Probably the best way is to double the 60, making 120 and then adding 10 (which is double 5) to make 170. Some children prefer to double the 5 first, but as this always gives us 10 I prefer to do the tens part first, knowing that I have just got to add 10 to the answer.

This is probably most suited to year 3, although many older children will benefit from the practice.

Double multiples of 5

Year 3 maths worksheet: Double 2-digit numbers

Doubling 2-digit numbers can be quite easy or fairly tricky; it all depends on the numbers. For a number such as 23, doubling is easy as the tens and units can be doubled in your head in either order and can be easily remembered.

However, doubling 37 leads to more complex mathematical thinking as at least three separate operations are needed to reach the correct answer. Probably the easiest way is to double the 30 to make 60. Holding 60 in the back of your mind add the 7 to make 67 and then add the 7 again to make 74. Another way is to double the 30 to make 60 and then double the 7 to make 14 and then add the 60 and the 14 to make 74.

There are other ways, including doubling 40 to make 80 and taking 6 away (double 3) to reach 74. It is always a good idea to see how children tackle these types of questions, ask them what they did and see if it is an efficient way.

Doubles 1

Year 4 maths worksheet: Doubling and halving

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.

This page can be found in the year 4 worksheets section.

Double multiples of 5