# Lesson 13

Put It All Together: Add and Subtract Fractions

### Lesson Purpose

The purpose of this lesson is for students to consider different denominators they can use to add or subtract fractions.

### Lesson Narrative

In this lesson, students examine and implement different strategies to find a common denominator when adding or subtracting fractions. For any pair of fractions, the product of the denominators will be a common denominator and in many cases this is also the smallest choice of common denominator. In other cases, however, there is a smaller choice than the product and sometimes this can be a useful choice. For example, the sum $$\frac{3}{20} + \frac{9}{50}$$ can be found using 1,000 as a common denominator but it can also be found with 100 as a common denominator. Either choice will work and there are reasons for preferring each strategy. In this lesson, students think about these ideas as they investigate different common denominators for finding sums and differences of fractions.

• Engagement
• MLR8

### Learning Goals

Teacher Facing

• Add and subtract fractions with unlike denominators.

### Student Facing

• Let’s add and subtract fractions with unlike denominators.

### 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

As students shared their ideas today, how did you ensure all students’ voices were heard and valued as an important part of the collective learning?