Lesson objective: Prove that changing the order of the addends does not change the sum.
This lesson helps to build fluency with adding three numbers in any order. A number line is used here because it supports the understanding that adding numbers in any order will give you the same sum. This work develops students' understanding that we can add numbers in any order.
Students engage in Mathematical Practice 7 (Look for and make use of structure) as they change the order of the addends to help them find the sum.
Key vocabulary:
 addend: one of the numbers in an indicated sum of two or more numbers (e.g., 4 + 3 + 1 = 8; 4, 3, and 1 are addends)
 number line: a line on which a point represents a value or number

order: to place numbers or objects in a sequential arrangement (e.g., least to greatest or heaviest to lightest)

sum: the result when two or more quantities are added
 total: the whole amount or value
Special materials needed:
 number lines (020)