Add and Subtract Fractions with a Common Denominator

Activity Overview

Before starting this activity, revisit what “numerator” and “denominator” mean, stressing that the denominator gives the name to the fraction. When adding fractions, you are adding parts; as long as you are adding the same kind of parts (common denominator), you only need to worry about how many parts in all. In this activity, students will add or subtract two fractions using visuals to help explain the process.

Give students examples of when you might add parts together, but be careful of how you word your questions!

“Dennis ate 2 pieces of pizza and Larry ate 2 pieces of pizza. How much pizza did they eat?”

Is the answer four pieces? Four-eighths? All of the pizza?

Be explicit in the question or example. Identify how many pieces make up the whole. Use precise language when asking the question, such as “what fraction” or “how many pieces”, so students know what they are looking for.

By rewording the story, you can use the same example for subtraction. You can change the question to “what fraction of the pizza is left” or “how many pieces are left?”

Just as in subtraction with whole numbers, be sure students are writing the numbers in the correct order. Typically, students have not yet encountered improper fractions, and certainly not negative numbers. The larger fraction goes first in the number sentence: 3/4 - 1/4 = 2/4.