Addition is the inverse of subtraction
When working with addition and subtraction it is useful to understand that they are the inverse of each other. That means that addition can be used to undo subtraction and vice versa. For example, 34 + 27 = 61, 61 – 34 = 27 and 61 – 27 = 34.
This is also helpful when used to check answers to calculations.
Growing addition and subtraction
Once a strategy is mastered, larger numbers can be used. For example, if we know that 65 + 15 = 80, it isn't much of a jump to work out that 650 + 150 = 800.