Introduction

Before we get started, here is some important terminology that will help you understand the concepts about working with fractions in this section.

• product: the result of  multiplication
• factor: something being multiplied – for  [latex]3 \cdot 2 = 6[/latex] , both 3 and 2 are factors of 6
• numerator: the top part of a fraction – the numerator in the fraction [latex]\frac{2}{3}[/latex] is 2
• denominator: the bottom part of a fraction – the denominator in the fraction [latex]\frac{2}{3}[/latex] is 3

Note About Instructions

Many different words are used by math textbooks and teachers to provide students with instructions on what they are to do with a given problem. For example, you may see instructions such as “Find” or “Simplify” in the examples in this module. It is important to understand what these words mean so you can successfully work through the problems in this course. Here is a short list of the words you may see that can help you know how to work through the problems in this module.

Simplify a fraction to its lowest terms

Often, if the answer to a problem is a fraction, you will be asked to write it in lowest terms. This is a common convention used in mathematics, similar to starting a sentence with a capital letter and ending it with a period. In this course, we will not go into great detail about methods for reducing fractions because there are many. The process of simplifying a fraction is often called reducing the fraction. We can simplify by canceling (dividing) the common factors in a fraction’s numerator and denominator.  We can do this because a fraction represents division.

Rewriting fractions with the smallest factors possible is often called prime factorization.

Add fractions

When you need to add or subtract fractions, you will need to first make sure that the fractions have the same denominator. The denominator tells you how many pieces the whole has been broken into, and the numerator tells you how many of those pieces you are using.

The “parts of a whole” concept can be modeled with pizzas and pizza slices. For example, imagine a pizza is cut into 4 pieces, and someone takes 1 piece. Now, [latex]\frac{1}{4}[/latex] of the pizza is gone and [latex]\frac{3}{4}[/latex] remains. Note that both of these fractions have a denominator of 4, which refers to the number of slices the whole pizza has been cut into. What if you have another pizza that had been cut into 8 equal parts and 3 of those parts were gone, leaving [latex]\frac{5}{8}[/latex]?

Find the common denominator of two or more fractions

How can you describe the total amount of pizza that is left with one number rather than two different fractions? You need a common denominator, technically called the least common multiple. Remember that if a number is a multiple of another, you can divide them and have no remainder.

One way to find the least common multiple of two or more numbers is to first multiply each by 1, 2, 3, 4, etc.  For example, find the least common multiple of 2 and 5.

The smallest multiple they have in common will be the common denominator for the two!

Use the common denominator to add unlike fractions

To add fractions with unlike denominators, first rewrite them with like denominators. Then, you know what to do! The steps are shown below.

In the next example you are shown how to add two fractions with different denominators, then simplify the answer.

You can find a common denominator by finding the common multiples of the denominators. The least common multiple is the easiest to use.

In the following video you will see an example of how to add two fractions with different denominators.

You can also add more than two fractions as long as you first find a common denominator for all of them. An example of a sum of three fractions is shown below. In this example, you will use the prime factorization method to find the LCM.

Subtract fractions

When you subtract fractions, you must think about whether they have a common denominator, just like with adding fractions. Below are some examples of subtracting fractions whose denominators are not alike.

The example below shows how to use multiples to find the least common multiple, which will be the least common denominator.

In the following video you will see an example of how to subtract fractions with unlike denominators.

Multiply fractions

Just as you add, subtract, multiply, and divide when working with whole numbers, you also use these operations when working with fractions.   There are many times when it is necessary to multiply fractions. A model may help you understand multiplication of fractions.

When you multiply a fraction by a fraction, you are finding a “fraction of a fraction.” Suppose you have [latex]\frac{3}{4}[/latex] of a candy bar and you want to find [latex]\frac{1}{2}[/latex] of the [latex]\frac{3}{4}[/latex]:

By dividing each fourth in half, you can divide the candy bar into eighths.

Then, choose half of those to get [latex]\frac{3}{8}[/latex].

In both of the above cases, to find the answer, you can multiply the numerators together and the denominators together.

To review: if a fraction has common factors in the numerator and denominator, we can reduce the fraction to its simplified form by removing the common factors.

For example,

• Given [latex]\frac{8}{15}[/latex], the factors of 8 are: 1, 2, 4, 8 and the factors of 15 are: 1, 3, 5, 15.  [latex]\frac{8}{15}[/latex] is simplified because there are no common factors of 8 and 15.
• Given [latex]\frac{10}{15}[/latex], the factors of 10 are: 1, 2, 5, 10 and the factors of15 are: 1, 3, 5, 15. [latex]\frac{10}{15}[/latex] is not simplified because 5 is a common factor of 10 and 15.

You can simplify first, before you multiply two fractions, to make your work easier. This allows you to work with smaller numbers when you multiply.

In the following video you will see an example of how to multiply two fractions, then simplify the answer.

Divide fractions

There are times when you need to use division to solve a problem. For example, if painting one coat of paint on the walls of a room requires 3 quarts of paint and you have a bucket that contains 6 quarts of paint, how many coats of paint can you paint on the walls? You divide 6 by 3 for an answer of 2 coats. There will also be times when you need to divide by a fraction. Suppose painting a closet with one coat only required [latex]\frac{1}{2}[/latex] quart of paint. How many coats could be painted with the 6 quarts of paint? To find the answer, you need to divide 6 by the fraction, [latex]\frac{1}{2}[/latex].

Before we begin dividing fractions, let’s cover some important terminology.

• reciprocal: two fractions are reciprocals if their product is 1 (Don’t worry; we will show you examples of what this means.)
• quotient: the result of division

Find the reciprocal of a number

Dividing fractions requires using the reciprocal of a number or fraction. If you multiply two numbers together and get 1 as a result, then the two numbers are reciprocals. Here are some examples of reciprocals:

Sometimes we call the reciprocal the “flip” of the other number: flip [latex]\frac{2}{5}[/latex] to get the reciprocal [latex]\frac{5}{2}[/latex].

Division with zero

You know what it means to divide by 2 or divide by 10, but what does it mean to divide a quantity by 0? Is this even possible? Can you divide 0 by a number? Consider the fraction

[latex]\frac{0}{8}[/latex]

We can read it as, “zero divided by eight.” Since multiplication is the inverse of division, we could rewrite this as a multiplication problem.

[latex]\text{?}\cdot{8}=0[/latex].

We can infer that the unknown must be 0 since that is the only number that will give a result of 0 when it is multiplied by 8.

Now let’s consider the reciprocal of [latex]\frac{0}{8}[/latex] which would be [latex]\frac{8}{0}[/latex]. If we rewrite this as a multiplication problem, we will have

[latex]\text{?}\cdot{0}=8[/latex].

This doesn’t make any sense. There are no numbers that you can multiply by zero to get a result of 8. The reciprocal of [latex]\frac{8}{0}[/latex] is undefined, and in fact, all division by zero is undefined.

Caution! Division by zero is undefined and so is the reciprocal of any fraction that has a zero in the numerator. For any real number a, [latex]\frac{a}{0}[/latex] is undefined. Additionally, the reciprocal of [latex]\frac{0}{a}[/latex] will always be undefined.

Divide a fraction by a whole number

When you divide by a whole number, you are multiplying by the reciprocal. In the painting example where you need 3 quarts of paint for a coat and have 6 quarts of paint, you can find the total number of coats that can be painted by dividing 6 by 3, [latex]6\div3=2[/latex]. You can also multiply 6 by the reciprocal of 3, which is [latex]\frac{1}{3}[/latex], so the multiplication problem becomes

[latex]\frac{6}{1}\cdot \frac{1}{3}=\frac{6}{3}=2[/latex].

The same idea will work when the divisor (the thing being divided) is a fraction. If you have [latex]\frac{3}{4}[/latex] of a candy bar and need to divide it among 5 people, each person gets [latex]\frac{1}{5}[/latex] of the available candy:

[latex]\frac{1}{5}\text{ of }\frac{3}{4}=\frac{1}{5}\cdot \frac{3}{4}=\frac{3}{20}[/latex]

Each person gets [latex]\frac{3}{20}[/latex] of a whole candy bar.

If you have a recipe that needs to be divided in half, you can divide each ingredient by 2, or you can multiply each ingredient by [latex]\frac{1}{2}[/latex] to find the new amount.

For example, dividing by 6 is the same as multiplying by the reciprocal of 6, which is [latex]\frac{1}{6}[/latex]. Look at the diagram of two pizzas below.  How can you divide what is left (the red shaded region) among 6 people fairly?

Each person gets one piece, so each person gets [latex]\frac{1}{4}[/latex] of a pizza.

Dividing a fraction by a whole number is the same as multiplying by the reciprocal, so you can always use multiplication of fractions to solve division problems.

Divide a fraction by a fraction

Sometimes you need to solve a problem that requires dividing by a fraction. Suppose you have a pizza that is already cut into 4 slices. How many [latex]\frac{1}{2}[/latex] slices are there?

There are 8 slices. You can see that dividing 4 by [latex]\frac{1}{2}[/latex] gives the same result as multiplying 4 by 2.

What would happen if you needed to divide each slice into thirds?

You would have 12 slices, which is the same as multiplying 4 by 3.

Any easy way to remember how to divide fractions is the phrase “keep, change, flip.” This means to KEEP the first number, CHANGE the division sign to multiplication, and then FLIP (use the reciprocal) of the second number.