Foundations

6 Visualize Fractions

Learning Objectives

By the end of this section, you will be able to:

  • Find equivalent fractions
  • Simplify fractions
  • Multiply fractions
  • Divide fractions
  • Simplify expressions written with a fraction bar
  • Translate phrases to expressions with fractions

A more thorough introduction to the topics covered in this section can be found in the Prealgebra chapter, Fractions.

Find Equivalent Fractions

Fractions are a way to represent parts of a whole. The fraction \frac{1}{3} means that one whole has been divided into 3 equal parts and each part is one of the three equal parts. See (Figure). The fraction \frac{2}{3} represents two of three equal parts. In the fraction \frac{2}{3}, the 2 is called the numerator and the 3 is called the denominator.

The circle on the left has been divided into 3 equal parts. Each part is \frac{1}{3} of the 3 equal parts. In the circle on the right, \frac{2}{3} of the circle is shaded (2 of the 3 equal parts).

Two circles are shown, each divided into three equal pieces by lines. The left hand circle is labeled “one third” in each section. Each section is shaded. The circle on the right is shaded in two of its three sections.

Doing the Manipulative Mathematics activity “Model Fractions” will help you develop a better understanding of fractions, their numerators and denominators.
Fraction

A fraction is written \frac{a}{b}, where b\ne 0 and

  • a is the numerator and b is the denominator.

A fraction represents parts of a whole. The denominator b is the number of equal parts the whole has been divided into, and the numerator a indicates how many parts are included.

If a whole pie has been cut into 6 pieces and we eat all 6 pieces, we ate \frac{6}{6} pieces, or, in other words, one whole pie.

A circle is shown and is divided into six section. All sections are shaded.

So \frac{6}{6}=1. This leads us to the property of one that tells us that any number, except zero, divided by itself is 1.

Property of One
\begin{array}{cccc}\frac{a}{a}=1\hfill & & & \left(a\ne 0\right)\hfill \end{array}

Any number, except zero, divided by itself is one.

Doing the Manipulative Mathematics activity “Fractions Equivalent to One” will help you develop a better understanding of fractions that are equivalent to one.

If a pie was cut in 6 pieces and we ate all 6, we ate \frac{6}{6} pieces, or, in other words, one whole pie. If the pie was cut into 8 pieces and we ate all 8, we ate \frac{8}{8} pieces, or one whole pie. We ate the same amount—one whole pie.

The fractions \frac{6}{6} and \frac{8}{8} have the same value, 1, and so they are called equivalent fractions. Equivalent fractions are fractions that have the same value.

Let’s think of pizzas this time. (Figure) shows two images: a single pizza on the left, cut into two equal pieces, and a second pizza of the same size, cut into eight pieces on the right. This is a way to show that \frac{1}{2} is equivalent to \frac{4}{8}. In other words, they are equivalent fractions.

Since the same amount is of each pizza is shaded, we see that \frac{1}{2} is equivalent to \frac{4}{8}. They are equivalent fractions.

A circle is shown that is divided into eight equal wedges by lines. The left side of the circle is a pizza with four sections making up the pizza slices. The right side has four shaded sections. Below the diagram is the fraction four eighths.

Equivalent Fractions

Equivalent fractions are fractions that have the same value.

How can we use mathematics to change \frac{1}{2} into \frac{4}{8}? How could we take a pizza that is cut into 2 pieces and cut it into 8 pieces? We could cut each of the 2 larger pieces into 4 smaller pieces! The whole pizza would then be cut into 8 pieces instead of just 2. Mathematically, what we’ve described could be written like this as \frac{1·4}{2·4}=\frac{4}{8}. See (Figure).

Cutting each half of the pizza into 4 pieces, gives us pizza cut into 8 pieces: \frac{1·4}{2·4}=\frac{4}{8}.

A circle is shown and is divided in half by a vertical black line. It is further divided into eighths by the addition of dotted red lines.

This model leads to the following property:

Equivalent Fractions Property

If a,b,c are numbers where b\ne 0,c\ne 0, then

\frac{a}{b}=\frac{a·c}{b·c}

If we had cut the pizza differently, we could get

An image shows three rows of fractions. In the first row are the fractions “1, times 2, divided by 2, times 2, equals two fourths”. Next to this is the word “so” and the fraction “one half, equals two fourths. The second row reads “1, times 3, divided by 2 times 3, equals three sixths”. Next to this is the word “so” and the fraction “one half equals, three sixths”. The third row reads “1 times 10, divided by 2 times 10, ten twentieths”. Next to this is the word “so” and the fraction “one half equals, ten twentieths”.

So, we say \frac{1}{2},\frac{2}{4},\frac{3}{6},\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}\frac{10}{20} are equivalent fractions.

Doing the Manipulative Mathematics activity “Equivalent Fractions” will help you develop a better understanding of what it means when two fractions are equivalent.

Find three fractions equivalent to \frac{2}{5}.

Solution

To find a fraction equivalent to \frac{2}{5}, we multiply the numerator and denominator by the same number. We can choose any number, except for zero. Let’s multiply them by 2, 3, and then 5.

A row of fractions reads “2 times 2, divided by 5 times 2, equals four tenths”. Next to this is “2, times 3, divided by 5 times 3, equals six fifteenths”. Next to this is “2 times 5, divided by 5 times 5, equals ten twenty-fifths”.


So, \frac{4}{10},\frac{6}{15},\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}\frac{10}{25} are equivalent to \frac{2}{5}.

Find three fractions equivalent to \frac{3}{5}.

\frac{6}{10},\frac{9}{15},\frac{12}{20}; answers may vary

Find three fractions equivalent to \frac{4}{5}.

\frac{8}{10},\frac{12}{15},\frac{16}{20}; answers may vary

Simplify Fractions

A fraction is considered simplified if there are no common factors, other than 1, in its numerator and denominator.

For example,

  • \frac{2}{3} is simplified because there are no common factors of 2 and 3.
  • \frac{10}{15} is not simplified because 5 is a common factor of 10 and 15.
Simplified Fraction

A fraction is considered simplified if there are no common factors in its numerator and denominator.

The phrase reduce a fraction means to simplify the fraction. We simplify, or reduce, a fraction by removing the common factors of the numerator and denominator. A fraction is not simplified until all common factors have been removed. If an expression has fractions, it is not completely simplified until the fractions are simplified.

In (Figure), we used the equivalent fractions property to find equivalent fractions. Now we’ll use the equivalent fractions property in reverse to simplify fractions. We can rewrite the property to show both forms together.

Equivalent Fractions Property

If a,b,c are numbers where b\ne 0,c\ne 0,

\text{then}\phantom{\rule{1em}{0ex}}\frac{a}{b}=\frac{a·c}{b·c}\phantom{\rule{1em}{0ex}}\text{and}\phantom{\rule{1em}{0ex}}\frac{a·c}{b·c}=\frac{a}{b}

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{32}{56}.

Solution
-\phantom{\rule{0.2em}{0ex}}\frac{32}{56}
Rewrite the numerator and denominator showing the common factors. .
Simplify using the equivalent fractions property. -\phantom{\rule{0.2em}{0ex}}\frac{4}{7}


Notice that the fraction -\phantom{\rule{0.2em}{0ex}}\frac{4}{7} is simplified because there are no more common factors.

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{42}{54}.

-\phantom{\rule{0.2em}{0ex}}\frac{7}{9}

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{45}{81}.

-\phantom{\rule{0.2em}{0ex}}\frac{5}{9}

Sometimes it may not be easy to find common factors of the numerator and denominator. When this happens, a good idea is to factor the numerator and the denominator into prime numbers. Then divide out the common factors using the equivalent fractions property.

How to Simplify a Fraction

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{210}{385}.

Solution

A table is shown with three columns and three rows. The first row of the left column reads “Step 1. Rewrite the numerator and denominator to show the common factors. If needed, use a factor tree”. Next to this in the middle column, it reads “rewrite 210 and 285 as the product of the primes. Next to this in the right column, it reads “negative 210 divided by 385.” Under this, is the equation “two times three times five times seven.” The five and 7 are blue and red respectively.The next row down reads “Step 2. Simplify using the equivalent fractions property by dividing out common factors.” Next to this in the middle column, it reads, “Mark the common factors 5 and 7.” Next to this in the right column, it has the equation 2 times, three times five, times seven over 5 times seven times 11. Both the 5 and the 7 are crossed out as common factors. Under this is the equation “negative two times 3 divided by 11.”The next row reads, “Step 3. Multiply the remaining factors, if necessary.” Next to this in the right column is negative six elevenths.

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{69}{120}.

-\phantom{\rule{0.2em}{0ex}}\frac{23}{40}

Simplify: -\phantom{\rule{0.2em}{0ex}}\frac{120}{192}.

-\phantom{\rule{0.2em}{0ex}}\frac{5}{8}

We now summarize the steps you should follow to simplify fractions.

Simplify a Fraction.
  1. Rewrite the numerator and denominator to show the common factors.
    If needed, factor the numerator and denominator into prime numbers first.
  2. Simplify using the equivalent fractions property by dividing out common factors.
  3. Multiply any remaining factors, if needed.

Simplify: \frac{5x}{5y}.

Solution
\frac{5x}{5y}
Rewrite showing the common factors, then divide out the common factors. .
Simplify. \frac{x}{y}

Simplify: \frac{7x}{7y}.

\frac{x}{y}

Simplify: \frac{3a}{3b}.

\frac{a}{b}

Multiply Fractions

Many people find multiplying and dividing fractions easier than adding and subtracting fractions. So we will start with fraction multiplication.

Doing the Manipulative Mathematics activity “Model Fraction Multiplication” will help you develop a better understanding of multiplying fractions.

We’ll use a model to show you how to multiply two fractions and to help you remember the procedure. Let’s start with \frac{3}{4}.

A rectangle made up of four squares in a row. The first three squares are shaded.

Now we’ll take \frac{1}{2} of \frac{3}{4}.

A rectangle made up of four squares in a row. The first three squares are shaded. The bottom halves of the first three squares are shaded darker with diagonal lines.

Notice that now, the whole is divided into 8 equal parts. So \frac{1}{2}·\frac{3}{4}=\frac{3}{8}.

To multiply fractions, we multiply the numerators and multiply the denominators.

Fraction Multiplication

If a,b,c\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d are numbers where b\ne 0\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d\ne 0, then

\frac{a}{b}·\frac{c}{d}=\frac{ac}{bd}

To multiply fractions, multiply the numerators and multiply the denominators.

When multiplying fractions, the properties of positive and negative numbers still apply, of course. It is a good idea to determine the sign of the product as the first step. In (Figure), we will multiply negative and a positive, so the product will be negative.

Multiply: -\phantom{\rule{0.2em}{0ex}}\frac{11}{12}·\frac{5}{7}.

Solution

The first step is to find the sign of the product. Since the signs are the different, the product is negative.

-\phantom{\rule{0.2em}{0ex}}\frac{11}{12}·\frac{5}{7}
Determine the sign of the product; multiply. -\phantom{\rule{0.2em}{0ex}}\frac{11·5}{12·7}
Are there any common factors in the numerator and the demoninator? No. -\phantom{\rule{0.2em}{0ex}}\frac{55}{84}

Multiply: -\phantom{\rule{0.2em}{0ex}}\frac{10}{28}·\frac{8}{15}.

-\phantom{\rule{0.2em}{0ex}}\frac{4}{21}

Multiply: -\phantom{\rule{0.2em}{0ex}}\frac{9}{20}·\frac{5}{12}.

-\phantom{\rule{0.2em}{0ex}}\frac{3}{16}

When multiplying a fraction by an integer, it may be helpful to write the integer as a fraction. Any integer, a, can be written as \frac{a}{1}. So, for example, 3=\frac{3}{1}.

Multiply: -\phantom{\rule{0.2em}{0ex}}\frac{12}{5}\left(-20x\right).

Solution

Determine the sign of the product. The signs are the same, so the product is positive.

-\phantom{\rule{0.2em}{0ex}}\frac{12}{5}\left(-20x\right)
Write 20x as a fraction. \frac{12}{5}\left(\frac{20x}{1}\right)
Multiply.
Rewrite 20 to show the common factor 5 and divide it out. .
Simplify. 48x

Multiply: \frac{11}{3}\left(-9a\right).

-33a

Multiply: \frac{13}{7}\left(-14b\right).

-2b

Divide Fractions

Now that we know how to multiply fractions, we are almost ready to divide. Before we can do that, that we need some vocabulary.

The reciprocal of a fraction is found by inverting the fraction, placing the numerator in the denominator and the denominator in the numerator. The reciprocal of \frac{2}{3} is \frac{3}{2}.

Notice that \frac{2}{3}·\frac{3}{2}=1. A number and its reciprocal multiply to 1.

To get a product of positive 1 when multiplying two numbers, the numbers must have the same sign. So reciprocals must have the same sign.

The reciprocal of -\phantom{\rule{0.2em}{0ex}}\frac{10}{7} is -\phantom{\rule{0.2em}{0ex}}\frac{7}{10}, since -\phantom{\rule{0.2em}{0ex}}\frac{10}{7}\left(-\phantom{\rule{0.2em}{0ex}}\frac{7}{10}\right)=1.

Reciprocal

The reciprocal of \frac{a}{b} is \frac{b}{a}.

A number and its reciprocal multiply to one \frac{a}{b}·\frac{b}{a}=1.

Doing the Manipulative Mathematics activity “Model Fraction Division” will help you develop a better understanding of dividing fractions.

To divide fractions, we multiply the first fraction by the reciprocal of the second.

Fraction Division

If a,b,c\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d are numbers where b\ne 0,c\ne 0\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d\ne 0, then

\frac{a}{b}÷\frac{c}{d}=\frac{a}{b}·\frac{d}{c}

To divide fractions, we multiply the first fraction by the reciprocal of the second.

We need to say b\ne 0,c\ne 0\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d\ne 0 to be sure we don’t divide by zero!

Divide: -\phantom{\rule{0.2em}{0ex}}\frac{2}{3}÷\frac{n}{5}.

Solution
-\phantom{\rule{0.2em}{0ex}}\frac{2}{3}÷\frac{n}{5}
To divide, multiply the first fraction by the reciprocal of the second. -\phantom{\rule{0.2em}{0ex}}\frac{2}{3}·\frac{5}{n}
Multiply. -\phantom{\rule{0.2em}{0ex}}\frac{10}{3n}

Divide: -\phantom{\rule{0.2em}{0ex}}\frac{3}{5}÷\frac{p}{7}.

-\phantom{\rule{0.2em}{0ex}}\frac{21}{5p}

Divide: -\phantom{\rule{0.2em}{0ex}}\frac{5}{8}÷\frac{q}{3}.

-\phantom{\rule{0.2em}{0ex}}\frac{15}{8q}

Find the quotient: -\phantom{\rule{0.2em}{0ex}}\frac{7}{8}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{14}{27}\right).

Solution
-\phantom{\rule{0.2em}{0ex}}\frac{7}{18}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{14}{27}\right)
To divide, multiply the first fraction by the reciprocal of the second. -\phantom{\rule{0.2em}{0ex}}\frac{7}{18}\cdot -\phantom{\rule{0.2em}{0ex}}\frac{27}{14}
Determine the sign of the product, and then multiply.. \frac{7\cdot 27}{18\cdot 14}
Rewrite showing common factors. .
Remove common factors. \frac{3}{2\cdot 2}
Simplify. \frac{3}{4}

Find the quotient: -\phantom{\rule{0.2em}{0ex}}\frac{7}{27}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{35}{36}\right).

\frac{4}{15}

Find the quotient: -\phantom{\rule{0.2em}{0ex}}\frac{5}{14}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{15}{28}\right).

\frac{2}{3}

There are several ways to remember which steps to take to multiply or divide fractions. One way is to repeat the call outs to yourself. If you do this each time you do an exercise, you will have the steps memorized.

  • “To multiply fractions, multiply the numerators and multiply the denominators.”
  • “To divide fractions, multiply the first fraction by the reciprocal of the second.”

Another way is to keep two examples in mind:

This is an image with two columns. The first column reads “One fourth of two pizzas is one half of a pizza. Below this are two pizzas side-by-side with a line down the center of each one representing one half. The halves are labeled “one half”. Under this is the equation “2 times 1 fourth”. Under this is another equation “two over 1 times 1 fourth.” Under this is the fraction two fourths and under this is the fraction one half. The next column reads “there are eight quarters in two dollars.” Under this are eight quarters in two rows of four. Under this is the fraction equation 2 divided by one fourth. Under this is the equation “two over one divided by one fourth.” Under this is two over one times four over one. Under this is the answer “8”.

The numerators or denominators of some fractions contain fractions themselves. A fraction in which the numerator or the denominator is a fraction is called a complex fraction.

Complex Fraction

A complex fraction is a fraction in which the numerator or the denominator contains a fraction.

Some examples of complex fractions are:

\frac{\frac{6}{7}}{3}\phantom{\rule{1em}{0ex}}\frac{\frac{3}{4}}{\frac{5}{8}}\phantom{\rule{1em}{0ex}}\frac{\frac{x}{2}}{\frac{5}{6}}

To simplify a complex fraction, we remember that the fraction bar means division. For example, the complex fraction \frac{\frac{3}{4}}{\frac{5}{8}} means \frac{3}{4}÷\frac{5}{8}.

Simplify: \frac{\frac{3}{4}}{\frac{5}{8}}.

Solution
\frac{\frac{3}{4}}{\frac{5}{8}}
Rewrite as division. \frac{3}{4}÷\frac{5}{8}
Multiply the first fraction by the reciprocal of the second. \frac{3}{4}\cdot \frac{8}{5}
Multiply. \frac{3\cdot 8}{4\cdot 5}
Look for common factors. .
Divide out common factors and simplify. \frac{6}{5}

Simplify: \frac{\frac{2}{3}}{\frac{5}{6}}.

\frac{4}{5}

Simplify: \frac{\frac{3}{7}}{\frac{6}{11}}.

\frac{11}{14}

Simplify: \frac{\frac{x}{2}}{\frac{xy}{6}}.

Solution
\frac{\frac{x}{2}}{\frac{xy}{6}}
Rewrite as division. \frac{x}{2}÷\frac{xy}{6}
Multiply the first fraction by the reciprocal of the second. \frac{x}{2}\cdot \frac{6}{xy}
Multiply. \frac{x\cdot 6}{2\cdot xy}
Look for common factors. .
Divide out common factors and simplify. \frac{3}{y}

Simplify: \frac{\frac{a}{8}}{\frac{ab}{6}}.

\frac{3}{4b}

Simplify: \frac{\frac{p}{2}}{\frac{pq}{8}}.

\frac{4}{2q}

Simplify Expressions with a Fraction Bar

The line that separates the numerator from the denominator in a fraction is called a fraction bar. A fraction bar acts as grouping symbol. The order of operations then tells us to simplify the numerator and then the denominator. Then we divide.

To simplify the expression \frac{5-3}{7+1}, we first simplify the numerator and the denominator separately. Then we divide.

\frac{5-3}{7+1}
\frac{2}{8}
\frac{1}{4}
Simplify an Expression with a Fraction Bar.
  1. Simplify the expression in the numerator. Simplify the expression in the denominator.
  2. Simplify the fraction.

Simplify: \frac{4-2\left(3\right)}{{2}^{2}+2}.

Solution

\begin{array}{cccccc}& & & & & \hfill \frac{4-2\left(3\right)}{{2}^{2}+2}\hfill \\ \begin{array}{c}\text{Use the order of operations to simplify the}\hfill \\ \text{numerator and the denominator.}\hfill \end{array}\hfill & & & & & \hfill \frac{4-6}{4+2}\hfill \\ \text{Simplify the numerator and the denominator.}\hfill & & & & & \hfill \frac{-2}{6}\hfill \\ \begin{array}{c}\text{Simplify. A negative divided by a positive is}\hfill \\ \text{negative.}\hfill \end{array}\hfill & & & & & \hfill -\phantom{\rule{0.2em}{0ex}}\frac{1}{3}\hfill \end{array}

\frac{4-2\left(3\right)}{{2}^{2}+2}
Use the order of operations to simpliy the numerator and the denominator. \frac{4-6}{4+2}
Simplify the numerator and the denominator. \frac{-2}{6}
Simplify. A negative divided by a positive is negative. -\phantom{\rule{0.2em}{0ex}}\frac{1}{3}

Simplify: \frac{6-3\left(5\right)}{{3}^{2}+3}.

-\phantom{\rule{0.2em}{0ex}}\frac{3}{4}

Simplify: \frac{4-4\left(6\right)}{{3}^{2}+3}.

-\phantom{\rule{0.2em}{0ex}}\frac{2}{3}

Where does the negative sign go in a fraction? Usually the negative sign is in front of the fraction, but you will sometimes see a fraction with a negative numerator, or sometimes with a negative denominator. Remember that fractions represent division. When the numerator and denominator have different signs, the quotient is negative.

\begin{array}{cccccc}\frac{-1}{3}=-\phantom{\rule{0.2em}{0ex}}\frac{1}{3}\hfill & & & & & \frac{\text{negative}}{\text{positive}}=\text{negative}\hfill \\ \frac{1}{-3}=-\phantom{\rule{0.2em}{0ex}}\frac{1}{3}\hfill & & & & & \frac{\text{positive}}{\text{negative}}=\text{negative}\hfill \end{array}
Placement of Negative Sign in a Fraction

For any positive numbers a and b,

\frac{\text{−}a}{b}=\frac{a}{\text{−}b}=-\phantom{\rule{0.2em}{0ex}}\frac{a}{b}

Simplify: \frac{4\left(-3\right)+6\left(-2\right)}{-3\left(2\right)-2}.

Solution
\begin{array}{c}& \frac{4\left(-3\right)+6\left(-2\right)}{-3\left(2\right)-2}\end{array}
Multiply. \frac{-12+\left(-12\right)}{-6-2}
Simplify. \frac{-24}{-8}
Divide. 3

Simplify: \frac{8\left(-2\right)+4\left(-3\right)}{-5\left(2\right)+3}.

4

Simplify: \frac{7\left(-1\right)+9\left(-3\right)}{-5\left(3\right)-2}.

2

Translate Phrases to Expressions with Fractions

Now that we have done some work with fractions, we are ready to translate phrases that would result in expressions with fractions.

The English words quotient and ratio are often used to describe fractions. Remember that “quotient” means division. The quotient of a and b is the result we get from dividing a by b, or \frac{a}{b}.

Translate the English phrase into an algebraic expression: the quotient of the difference of m and n, and p.

Solution

We are looking for the quotient of the difference of m and n, and p. This means we want to divide the difference of m\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}n\phantom{\rule{0.2em}{0ex}}\text{by}\phantom{\rule{0.2em}{0ex}}p.

\frac{m-n}{p}

Translate the English phrase into an algebraic expression: the quotient of the difference of a and b, and cd.

\frac{a-b}{cd}

Translate the English phrase into an algebraic expression: the quotient of the sum of p and q, and r

\frac{p+q}{r}

Key Concepts

  • Equivalent Fractions Property: If a,b,c are numbers where b\ne 0,c\ne 0, then
    \frac{a}{b}=\frac{a·c}{b·c} and \frac{a·c}{b·c}=\frac{a}{b}.
  • Fraction Division: If a,b,c\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d are numbers where b\ne 0,c\ne 0,\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d\ne 0, then \frac{a}{b}÷\frac{c}{d}=\frac{a}{b}·\frac{d}{c}. To divide fractions, multiply the first fraction by the reciprocal of the second.
  • Fraction Multiplication: If a,b,c\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d are numbers where b\ne 0,\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}d\ne 0, then \frac{a}{b}·\frac{c}{d}=\frac{ac}{bd}. To multiply fractions, multiply the numerators and multiply the denominators.
  • Placement of Negative Sign in a Fraction: For any positive numbers a\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}b, \frac{-a}{b}=\frac{a}{-b}=-\phantom{\rule{0.2em}{0ex}}\frac{a}{b}.
  • Property of One:\frac{a}{a}=1; Any number, except zero, divided by itself is one.
  • Simplify a Fraction
    1. Rewrite the numerator and denominator to show the common factors. If needed, factor the numerator and denominator into prime numbers first.
    2. Simplify using the equivalent fractions property by dividing out common factors.
    3. Multiply any remaining factors.
  • Simplify an Expression with a Fraction Bar
    1. Simplify the expression in the numerator. Simplify the expression in the denominator.
    2. Simplify the fraction.

Practice Makes Perfect

Find Equivalent Fractions

In the following exercises, find three fractions equivalent to the given fraction. Show your work, using figures or algebra.

\frac{3}{8}

\frac{6}{16},\frac{9}{24},\frac{12}{32} answers may vary

\frac{5}{8}

\frac{5}{9}

\frac{10}{18},\frac{15}{27},\frac{20}{36} answers may vary

\frac{1}{8}

Simplify Fractions

In the following exercises, simplify.

-\phantom{\rule{0.2em}{0ex}}\frac{40}{88}

-\phantom{\rule{0.2em}{0ex}}\frac{5}{11}

-\phantom{\rule{0.2em}{0ex}}\frac{63}{99}

-\phantom{\rule{0.2em}{0ex}}\frac{108}{63}

-\phantom{\rule{0.2em}{0ex}}\frac{12}{7}

-\phantom{\rule{0.2em}{0ex}}\frac{104}{48}

\frac{120}{252}

\frac{10}{21}

\frac{182}{294}

-\phantom{\rule{0.2em}{0ex}}\frac{3x}{12y}

-\phantom{\rule{0.2em}{0ex}}\frac{x}{4y}

-\phantom{\rule{0.2em}{0ex}}\frac{4x}{32y}

\frac{14{x}^{2}}{21y}

\frac{2{x}^{2}}{3y}

\frac{24a}{32{b}^{2}}

Multiply Fractions

In the following exercises, multiply.

\frac{3}{4}·\frac{9}{10}

\frac{27}{40}

\frac{4}{5}·\frac{2}{7}

-\phantom{\rule{0.2em}{0ex}}\frac{2}{3}\left(-\phantom{\rule{0.2em}{0ex}}\frac{3}{8}\right)

\frac{1}{4}

-\phantom{\rule{0.2em}{0ex}}\frac{3}{4}\left(-\phantom{\rule{0.2em}{0ex}}\frac{4}{9}\right)

-\phantom{\rule{0.2em}{0ex}}\frac{5}{9}·\frac{3}{10}

-\phantom{\rule{0.2em}{0ex}}\frac{1}{6}

-\phantom{\rule{0.2em}{0ex}}\frac{3}{8}·\frac{4}{15}

\left(-\phantom{\rule{0.2em}{0ex}}\frac{14}{15}\right)\left(\frac{9}{20}\right)

-\phantom{\rule{0.2em}{0ex}}\frac{21}{50}

\left(-\phantom{\rule{0.2em}{0ex}}\frac{9}{10}\right)\left(\frac{25}{33}\right)

\left(-\phantom{\rule{0.2em}{0ex}}\frac{63}{84}\right)\left(-\phantom{\rule{0.2em}{0ex}}\frac{44}{90}\right)

\frac{11}{30}

\left(-\phantom{\rule{0.2em}{0ex}}\frac{63}{60}\right)\left(-\phantom{\rule{0.2em}{0ex}}\frac{40}{88}\right)

4·\frac{5}{11}

\frac{20}{11}

5·\frac{8}{3}

\frac{3}{7}·21n

9n

\frac{5}{6}·30m

-8\left(\frac{17}{4}\right)

-34

\left(-1\right)\left(-\phantom{\rule{0.2em}{0ex}}\frac{6}{7}\right)

Divide Fractions

In the following exercises, divide.

\frac{3}{4}÷\frac{2}{3}

\frac{9}{8}

\frac{4}{5}÷\frac{3}{4}

-\phantom{\rule{0.2em}{0ex}}\frac{7}{9}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{7}{4}\right)

1

-\phantom{\rule{0.2em}{0ex}}\frac{5}{6}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{5}{6}\right)

\frac{3}{4}÷\frac{x}{11}

\frac{33}{4x}

\frac{2}{5}÷\frac{y}{9}

\frac{5}{18}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{15}{24}\right)

-\phantom{\rule{0.2em}{0ex}}\frac{4}{9}

\frac{7}{18}÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{14}{27}\right)

\frac{8u}{15}÷\frac{12v}{25}

\frac{10u}{9v}

\frac{12r}{25}÷\frac{18s}{35}

-5÷\frac{1}{2}

-10

-3÷\frac{1}{4}

\frac{3}{4}÷\left(-12\right)

-\phantom{\rule{0.2em}{0ex}}\frac{1}{16}

-15÷\left(-\phantom{\rule{0.2em}{0ex}}\frac{5}{3}\right)

In the following exercises, simplify.

\frac{-\phantom{\rule{0.2em}{0ex}}\frac{8}{21}}{\frac{12}{35}}

-\phantom{\rule{0.2em}{0ex}}\frac{10}{9}

\frac{-\phantom{\rule{0.2em}{0ex}}\frac{9}{16}}{\frac{33}{40}}

\frac{-\phantom{\rule{0.2em}{0ex}}\frac{4}{5}}{2}

-\phantom{\rule{0.2em}{0ex}}\frac{2}{5}

\frac{5}{\frac{3}{10}}

\frac{\frac{m}{3}}{\frac{n}{2}}

\frac{2m}{3n}

\frac{-\phantom{\rule{0.2em}{0ex}}\frac{3}{8}}{-\phantom{\rule{0.2em}{0ex}}\frac{y}{12}}

Simplify Expressions Written with a Fraction Bar

In the following exercises, simplify.

\frac{22+3}{10}

\frac{5}{2}

\frac{19-4}{6}

\frac{48}{24-15}

\frac{16}{3}

\frac{46}{4+4}

\frac{-6+6}{8+4}

0

\frac{-6+3}{17-8}

\frac{4·3}{6·6}

\frac{1}{3}

\frac{6·6}{9·2}

\frac{{4}^{2}-1}{25}

\frac{3}{5}

\frac{{7}^{2}+1}{60}

\frac{8·3+2·9}{14+3}

2\frac{8}{17}

\frac{9·6-4·7}{22+3}

\frac{5·6-3·4}{4·5-2·3}

\frac{3}{5}

\frac{8·9-7·6}{5·6-9·2}

\frac{{5}^{2}-{3}^{2}}{3-5}

-8>

\frac{{6}^{2}-{4}^{2}}{4-6}

\frac{7·4-2\left(8-5\right)}{9·3-3·5}

\frac{11}{6}

\frac{9·7-3\left(12-8\right)}{8·7-6·6}

\frac{9\left(8-2\right)-3\left(15-7\right)}{6\left(7-1\right)-3\left(17-9\right)}

\frac{5}{2}

\frac{8\left(9-2\right)-4\left(14-9\right)}{7\left(8-3\right)-3\left(16-9\right)}

Translate Phrases to Expressions with Fractions

In the following exercises, translate each English phrase into an algebraic expression.

the quotient of r and the sum of s and 10

\frac{r}{s+10}

the quotient of A and the difference of 3 and B

the quotient of the difference of x\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}y,\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}-3

\frac{x-y}{-3}

the quotient of the sum of m\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}n,\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}4q

Everyday Math

Baking. A recipe for chocolate chip cookies calls for \frac{3}{4} cup brown sugar. Imelda wants to double the recipe. How much brown sugar will Imelda need? Show your calculation. Measuring cups usually come in sets of \frac{1}{4},\frac{1}{3},\frac{1}{2},\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}1 cup. Draw a diagram to show two different ways that Imelda could measure the brown sugar needed to double the cookie recipe.

1\frac{1}{2} cups answers will vary

Baking. Nina is making 4 pans of fudge to serve after a music recital. For each pan, she needs \frac{2}{3} cup of condensed milk. How much condensed milk will Nina need? Show your calculation. Measuring cups usually come in sets of \frac{1}{4},\frac{1}{3},\frac{1}{2},\phantom{\rule{0.2em}{0ex}}\text{and}\phantom{\rule{0.2em}{0ex}}1 cup. Draw a diagram to show two different ways that Nina could measure the condensed milk needed for 4 pans of fudge.

Portions Don purchased a bulk package of candy that weighs 5 pounds. He wants to sell the candy in little bags that hold \frac{1}{4} pound. How many little bags of candy can he fill from the bulk package?

20 bags

Portions Kristen has \frac{3}{4} yards of ribbon that she wants to cut into 6 equal parts to make hair ribbons for her daughter’s 6 dolls. How long will each doll’s hair ribbon be?

Writing Exercises

Rafael wanted to order half a medium pizza at a restaurant. The waiter told him that a medium pizza could be cut into 6 or 8 slices. Would he prefer 3 out of 6 slices or 4 out of 8 slices? Rafael replied that since he wasn’t very hungry, he would prefer 3 out of 6 slices. Explain what is wrong with Rafael’s reasoning.

Answers may vary

Give an example from everyday life that demonstrates how \frac{1}{2}·\frac{2}{3}\phantom{\rule{0.2em}{0ex}}\text{is}\phantom{\rule{0.2em}{0ex}}\frac{1}{3}.

Explain how you find the reciprocal of a fraction.

Answers may vary

Explain how you find the reciprocal of a negative number.

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

A table is shown that is made up of four columns and seven rows. The first row reads “I can…” in the first column, “Confidently” in the second column, “With some help” in the third column and “No – I don’t get it” in the last column. The next row down in the first column reads “find equivalent fractions”, under this reads “simplify fractions”, under this reads “multiply fractions”, under this reads “divide fractions”, under this reads “Simplify expressions written with a fraction bar” and under this reads “translate phrases to expressions with fractions.”

After looking at the checklist, do you think you are well prepared for the next section? Why or why not?

Glossary

complex fraction
A complex fraction is a fraction in which the numerator or the denominator contains a fraction.
denominator
The denominator is the value on the bottom part of the fraction that indicates the number of equal parts into which the whole has been divided.
equivalent fractions
Equivalent fractions are fractions that have the same value.
fraction
A fraction is written \frac{a}{b}, where b\ne 0 a is the numerator and b is the denominator. A fraction represents parts of a whole. The denominator b is the number of equal parts the whole has been divided into, and the numerator a indicates how many parts are included.
numerator
The numerator is the value on the top part of the fraction that indicates how many parts of the whole are included.
reciprocal
The reciprocal of \frac{a}{b} is \frac{b}{a}. A number and its reciprocal multiply to one: \frac{a}{b}·\frac{b}{a}=1.
simplified fraction
A fraction is considered simplified if there are no common factors in its numerator and denominator.

License

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Elementary Algebra by OSCRiceUniversity is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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