Students first learn about addition to 20 using techniques such as counting on, making-ten and number bonds. After they know the techniques, they should start to recall basic facts for addition to 20 fluently.
One of the facts for addition to 20 that students often find easier to learn are the doubles, e.g. 6+6=12, 7+7=14, etc.
Once they are fluent in doubles, we can build on the known facts and introduce derived facts using doubles. For example, 7+7=14, so 7+8 is one more than 14, which is 15.
For more related resources, please see our Number Sense, Addition and Subtraction page.