Lesson 10

All Sorts of Denominators

Lesson Purpose

The purpose of this lesson is for students to explain why it is possible to find a common denominator for two given fractions by multiplying the denominators. 

Lesson Narrative

In this lesson students begin to develop formal strategies for adding and subtracting fractions. They deal with all cases, including situations where the two denominators share no common factor and cases where the denominators share a large factor. Students show why the product of the two denominators is always a common denominator but are encouraged to use the common denominator that makes sense to them. The discussions focus on different choices students make for a common denominator and how their calculations are the same and different.

  • Representation
  • MLR8

Learning Goals

Teacher Facing

  • Recognize that when adding or subtracting fractions with unlike denominators, a common denominator can be found by multiplying the denominators.

Student Facing

  •  Let’s find common denominators.

Required Preparation

CCSS Standards


Lesson Timeline

Warm-up 10 min
Activity 1 15 min
Activity 2 20 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

  • How Close? (1–5), Stage 9: Add Fractions to 5 (Addressing)
  • Compare (1–5), Stage 6: Add and Subtract Fractions (Supporting)

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Additional Resources

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