In this lesson, students use an addition family to write addition number sentences when the “whole” is eleven through twenty. They are given a number family from which they write all the fact addition sentences for that fact family. When the two numbers added are different, there are two addition sentences. When the two numbers added are the same, there is one addition sentence.
In this lesson, students learn to find the missing number in a number family when the “whole” is eleven through twenty and one of the numbers is unknown. When the missing number is the number that represents the sum, the student must add the other two numbers to find the missing sum. When the missing number is one of the numbers that make up the sum, the student must subtract the known smaller number from the sum to find the missing number.