Use double and double again (x4)

 

In this activity, we will learn to use a quick number trick to multiply numbers by four.

 

Learning the four times table is important, but sometimes mental calculations can be useful to find multiplication facts too. 

So, we will be using the skills of doubling to help find the answers to questions where we need to multiply by four. 

Let's look at an example.

 

Here are five candles.

 

If we double this number we get 2 × 5 = 10 candles

       

 

If we double this number again we get 2 × 10 = 20 candles

                                 

 

This is 4 lots of the original 5 candles so 4 × 5 = 20

 

So, to multiply a number by 4, we double it and double the answer again. 

 

Let's try an example.

 

Example

Find 4 × 5 by using this table to find first A and then B.

 

Double 5  is			A
Double	A	is	B
Then 4 × 5  is			B

 

Answer

Double 5 is 10, so A is 10.

Double 10 is 20, so B is 20.

So 4 × 5 is 20.

 

Now, we've looked at the method, you are ready to try some questions in the same style.