Lesson objective: Fluently add and subtract decimals using the standard algorithm.

This lesson helps to build fluency with adding and subtracting decimals. The standard algorithm is used here because it is the most efficient method for operations. This work develops students' understanding that rules for all operations with whole numbers and decimals are based on the place value system.

Students engage in Mathematical Practice 2 (Reason abstractly and quantitatively) as they notice how the placement of a digit in the algorithm, and in relation to the other numbers within the algorithm, gives it its place value - that the “place” it is in determines its “value”. Students engage in Mathematical Practice 7 (Look for and make use of structure) as they understand how the structure of the standard algorithm shows place value of each of the digits within the algorithm.

Teachers should frequently question students so they can articulate understanding of the algorithm. "Why is it important to line up decimals?" (*So the place values are in line.) *Encourage students to use zeros as place holders or to turn their paper so the lines for a space for each digit.

**Key vocabulary:**

- standard algorithm