Use Double and Double Again To Solve Multiplications

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 skill of doubling to help to 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 first find 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.