Lesson 13 Put It All Together: Add and Subtract Fractions

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

Warm-up Number Talk: Sums with $\frac{1}{8}$

Find the value of each expression mentally.

$\frac{1}{8} + \frac{5}{8}$
$\frac{1}{8} + \frac{6}{16}$
$\frac{1}{8} + \frac{1}{3}$
$\frac{1}{8} + \frac{5}{12}$

Activity 1 Common Denominators

$\frac{4}{6} + \frac{5}{8}$

Tyler says: “To find the sum, I can use 18 as a common denominator.”

Han says: “To find the sum, I can use 24 as a common denominator.”

Clare says: “To find the sum, I can use 48 as a common denominator.”

Whom do you agree with? Explain or show your reasoning.
What is the value of $\frac{4}{6} + \frac{5}{8}$ ?
Are there other common denominators you could use to find the sum? Explain or show your reasoning.

Activity 2 Unlike Denominators

Find the value of each expression. Explain or show your reasoning.

$\frac{2}{5} + \frac{13}{15}$
$\frac{6}{5} - \frac{1}{3}$
$\frac{11}{12} + 3 \frac{5}{9}$
$\frac{6}{10} - \frac{9}{25}$

Practice Problem

Problem 1

Find the value of each expression. Explain or show your reasoning.

$\frac{7}{8} + \frac{4}{13}$
$\frac{7}{8} - \frac{3}{20}$