Mental strategies for addition and subraction
 

How do you use ‘friendly’ numbers?

Most of us would much prefer to deal with 742 – 142 than 742 – 146, because the 142 and 742 are much more ‘friendly’ than the 146 and the 742. We always have as an option in addition and subtraction to use the compensation approach and temporarily replace one of the numbers in a calculation with one that is friendlier. This is especially useful in subtraction.

Calculate 742 – 146

Change the 146 to 142: 742 – 142 = 600

Now compensate: 742 – 146 = 600 – 4 = 596.

Or,

Change the 742 to 746: 746 – 146 = 600

Now compensate: 742 – 146 = 600 – 4 = 596.

Important:

Teach pupils specifically the strategies outlined in this module and give them opportunities to discuss different ways of tackling additions and subtractions by mental methods supported by jottings and empty number-line diagrams.