Using Adding and Subtracting Family Groups (1)

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