Math > Year 3

Multiplying mentally

Mentally means working out the calculations in our heads. 

 

Ways of multiplying mentally: 
 




 

Commutative property: 

The commutative property of multiplication states that the order of multiplying two numbers does not change the product (a × b = b × a).


Example: 4 × 6 and 6 × 4

 


 


Associative property:

The associative property of multiplication states that the product of three or more numbers remains the same regardless of how the numbers are grouped.  a × (b × c) = (a × b) × c

For example: 2 × (5 × 3) = (2 × 5) × 3
 



 


We can carry out the multiplication by using related multiplication facts.
 

Example: 20 × 3

To solve the above-given calculation, we can do 2 × 3 = 6

20 is 10 times larger than 2. So, the answer will also be 10 times bigger.

So, 20 × 3 = 60



Let’s try a question:
 

Question: Which of the following can be used to calculate 15 × 2 mentally?

1. 2 × 3 × 5 

2. 15 × 2 × 1

Explanation: 2 × 3 × 5 

We know that 15 = 3 × 5
Since 15 × 2 = 2 × 15, we can say that 15 × 2 = 2 × 3 × 5

So 2 × 3 × 5 can be used to solve 15 × 2 mentally.


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