Multiplication - the half and double method

Multiplication - the half and double method

Learning maths at primary school age is a little bit like collecting tools for a tool kit. 

Here's a super-useful tool that can come in handy to simplify some tricky looking multiplication.

Multiplying using the half and double method

When you're faced with a multiplication like:

4 x 16 = ?

You have a number of methods to use:

  1. Short Multiplication  - the written method we all learned at school - here's a video to remind you

  2. The Grid Method as discussed in this article

  3. The Distributive Law - which sounds scary but basically means "splitting" it into two easy multiplications - so 4 x 16 becomes (4 x 10 ) + (4 x 6 )

  4. Or sometimes, such as in this example, it's just perfect for the half and double method

Using the half and double method, we can halve one side of the multiplication as long as we double the other - and the answer remains the same.

So,

4 x 16 

becomes:

8 x 8 = 64


Here is another example which looks a little tricky at first:

34 x 5 = ?

But after using the half and double method it becomes:

17 x 10 = 170


The method also works very well with some fractions like this:

3½  x 12 = ?

Doubling removes the fraction so it becomes:

3½  x 12 =  7 x 6 = 42


Likewise with decimals:

4.5  x 8   =  9 x 4  = 36

 

The key to using the half and double strategy is choosing when it makes the problem easier. Here is a guide:

  • Is one of the sides 5? If so doubling will give 10 which is easy.
  • Is the side I need to halve even?
  • Does one side include a half or 0.5? If so doubling will remove it.

And that's it. A handy little multiplication strategy to add to your "maths toolkit." 

 

I'm Ged, Co-founder of Komodo, ex-maths teacher and dad. If you have any questions please get in touch.

About Komodo – Komodo is a fun and effective way to boost primary maths skills. Designed for 5 to 11-year-olds to use in the home, Komodo uses a little and often approach to learning maths (15 minutes, three to five times per week) that fits into the busy routine. Komodo users develop fluency and confidence in maths – without keeping them at the screen for long.

Find out more about Komodo and how it helps thousands of children each year do better at maths - you can even try Komodo for free.

Related Posts

How many times tables do you really have to learn?

Learning times tables can be really daunting for young children, so we've produced a video to show them that it's not as hard as they may think.

Starting primary school - how to make the first weeks go smoothly (advice from a teacher and other parents)

Starting school for the first time is a big milestone for every child, (and parent!). We asked a KS1 teacher and parents to share their tips on how to prepare for the first few weeks.