### Home > MC1 > Chapter 7 > Lesson 7.2.4 > Problem7-82

7-82.

Find common denominators and calculate each of the following sums.

1.  $\frac { 3 } { 8 } + \frac { 1 } { 4 }$

Remember, the denominator is the bottom number of a fraction. Here the denominators are $8$ and $4$. Can you change the fractions so that both denominators are $8$?

$\frac{5}{8}$

2.  $\frac { 2 } { 5 } + \frac { 1 } { 3 }$

Here, you might like to find the least common multiple of $5$ and $3$. This number should eventually be your common denominator.

$3$ and $5$ have a least common multiple of $15$.
Use multiplication to convert $\frac{2}{5}$ and $\frac{1}{3}$ to fractions with a denominator of $15$.
This will help you add the given fractions.

If $\frac{2}{5}+\frac{1}{3}=\frac{6}{15}+\frac{5}{15}$ , can you find the final sum?

3. What is the least common multiple of $8$ and $4$? Of $5$ and $3$?

Notice that $8$ is a multiple of $4$. Refer to part (b) for the least common multiple of $3$ and $5$.

4. Explain how finding the least common multiple of two numbers can help you add fractions.

Look back at parts (a), (b), and (c). How have least common multiples come in handy? Noticing this pattern can help you to explain your answer here.