# Lesson 13

### Lesson Purpose

The purpose of this lesson is for students to add decimals and consider common errors in lining up place values when adding with the standard algorithm.

### Lesson Narrative

In previous lessons, students used strategies based on place value and properties of operations to add decimal numbers including the standard algorithm. The purpose of this lesson is to highlight the importance of adding the same place values when making vertical calculations with decimals. This issue does not arise when adding whole numbers as the numbers are always aligned to the right, starting with ones, then tens, and so on. For decimal numbers it is more complicated because the last digits in two decimals, such as 5.18 and 16.7, may not have the same place value. Rather than aligning these numbers to the right as shown here

they need to be aligned by place value. After examining this alignment error, students practice adding decimals taking care to add using the correct place values.

• Engagement

### Learning Goals

Teacher Facing

• Add decimals to the hundredths place using strategies based on place value.

### Student Facing

• Let’s use place value strategies to add decimals.

### Lesson Timeline

 Warm-up 10 min Activity 1 20 min Activity 2 15 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

Reflect on a time your thinking changed about something in class recently. How will you alter your teaching practice to incorporate your new understanding?

### Suggested Centers

• Jump the Line (2–5), Stage 2: Add and Subtract Tenths and Hundredths (Supporting)
• How Close? (1–5), Stage 7: Multiply Fractions and Whole Numbers to 5 (Supporting)