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Addition and subtraction family facts normally come in FOURS.
There are TWO addition facts and TWO subtraction facts.
If we know just one of them, we can always work out the other three.
If we know the addition fact that
23 + 35 = 58
we can say that the other addition fact in this family is
35 + 23 = 58
The subtraction facts in this family will be
58 - 35 = 23
and
58 - 23 = 35
Example
Use the addition fact:
15 + 18 = 33
to work out the missing number below:
33 - 
= 15
Answer
15 + 18 = 33
So 33 - 18 = 15 and 33 - 15 = 18
The missing number is 18.
33 - 18 = 15
