Mental strategies for addition and subraction

Associative law of addition - part 2

We shall see below that deciding on the most efficient way of adding up various bits of numbers is an important strategy for informal calculations, so the associative and commutative laws of addition are important – even though most people use them without realising it.

An important point to make about associatively is that subtraction does not have this property. For example, 25 – (12-8) is not equal to (25 – 12) – 8. This means that we cannot write 25 – 12 – 8 to mean both of these! The convention is that 25 – 12 – 8 means (25 -12) – 8, that is that the subtractions are done in order from left to right, unless brackets are used to indicate otherwise.