Addition and Subtraction Facts To 20
Once children have mastered the addition and subtraction facts to 10, introduce the facts to 20. Encourage the use of cubes and fact cards. You will use many of the same strategies that you used in facts to 10 and add several new ones. Encourage children to share the strategies they use.
Materials: snap cubes, fact cards
Preparation: Prepare fact cards for addition facts through 20 and for the corresponding subtraction facts. Write the strategy on each card.
Prerequisite Skills and Concepts: Children should have a good foundation in the addition facts through 10 and the subtraction facts with minuends of 10 or less.
Have children connect what they learned about related facts through 10 to related facts to 20. Use only one or two sets of related facts a day.
- Say: We talked about related facts to 10. Today we are going to talk about related facts for 11 and 12 (or other numbers).
- Say: Make a train of 11 cubes. Cover 9 of the cubes. How many are still showing?
- Ask: What two facts can you write with those numbers?
9 + 2 = 11; 11 2 + 9. Repeat with other combinations for 11 and then 12.
- Say: We can also make the fact family using the same 3 numbers.
Have children make the 4 facts in that family.
- Ask: What four facts can you make using those 3 numbers?
9 + 2 = 11; 2 + 9 = 11; 11 2 + 9; 11 9 2. Repeat with other combinations. Be sure to include the doubles for 12, 14, 16, 18, and 20.
- Say: We know what doubles are. Today we are going to learn how to use doubles to help us find near doubles.
- Ask: Who can tell me what 5 + 5 is?
- Say: Yes, 5 + 5 = 10. We can now use that fact to help us find out what 5 + 6 is. 6 is one more than 5 so 5 + 6 is one more than 5 + 5.
- Ask: If 5 + 5 is 10, what is one more than 10?
Children should say 11.
- Say: Yes, 5 + 6 = 11. Now let's try some other doubles.
Repeat using other double and doubles-plus-one combinations.
Wrap-Up and Assessment Hints
Find out which facts a child has mastered and which facts the child needs help with. If a fact has not been mastered, ask a child how he or she might go about figuring it out. Find out what they are thinking and then select a strategy that builds on what they already know. Don't forget to encourage children to use number relationships and related facts and fact families.