Lesson 9

The purpose of this lesson is for students to use a number line to reason about subtraction of fractions with the same denominator.

Previously, students decomposed fractions into sums of other fractions and wrote equations to record the decompositions. They also used number lines to reason about addition of fractions with the same denominator. In this lesson, students use number lines to think about differences of fractions with the same denominator. They also practice reasoning about equivalence and decomposing fractions (including mixed numbers) mentally to facilitate subtraction.

As in earlier grades, students may think about subtracting as taking away a number from another number, or as finding an unknown addend. The way they represent differences of fractions on a number line may vary accordingly.

• Create a set of cards from the blackline master for each group of 2.

Based on students’ work today, what did you learn about their understanding of the relationships between addition and subtraction? How will you use these insights to prepare for upcoming work?