One of the most important concepts for children to understand in the early years is the relationship between addition and subtraction. Given one set of facts they should be able to deduce several other facts.
For example:
If 40 + 30 = 70 then:
30 + 40 = 70
and
70 – 40 = 30
and
70 – 30 = 40.
This free worksheet look at this relationship.