## Subtraction Facts Through 20

Once children have mastered addition and subtraction facts through 12, they apply the same strategies to learn facts through 20. Because addition and subtraction are inverse operations, knowing an addition fact can help find the differences for the subtraction facts that are part of the same fact family. Most fact families consist of four related facts, all of which use the same numbers.

For example, using the numbers 12, 4, and 16, four facts can be formed to make a family.

4 + 12 = 16

12 + 4 = 16

16 − 4 = 12

16 − 12 = 4

Using variables, the four members of the family can be written as

a + b = c

b + a = c

c − a = b

c − b = a

**Teaching Model 20.6:** Fact Families