Ratios & Proportions

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A cake divided into quarters, with one quarter removed, leaving three-quarters, or 3/4 of the cake

The word ratio is another word for fraction. In Mathematics, a fraction (from the Latin fractus, meaning broken) is a concept of a proportional relation between an object part and the object whole.

A fraction is an example of a specific type of ratio, in which the two numbers are related in a part-to-whole relationship, rather than as a comparative relation between two separate quantities.

A fraction is a quotient of numbers, the quantity obtained when the numerator is divided by the denominator. Thus 3/4 represents three divided by four, in decimals 0.75, as a percentage 75%. The three equal parts of the cake are 75% of the whole cake.

Each fraction consists of a denominator (bottom) and a numerator (top), representing (respectively) the number of equal parts that an object is divided into, and the number of those parts indicated for the particular fraction. Fractions are rational numbers, which means that the denominator and the numerator are integers.

For example, the fraction 3/4 could be used to represent three equal parts of a whole object, were it divided into four equal parts. Because it is impossible to divide something into zero equal parts, zero can never be the denominator of a fraction (see division by zero). A fraction with equal numerator and denominator is equal to one (e.g. 5/5 = 1) and the fraction form is rarely, if ever, given as a final result.

In higher mathematics, a fraction is viewed as an element of a field of fractions.

1 Forms of Fractions
2 Arithmetic with fractions

Forms of Fractions

Proper, and Improper Fractions

A common fraction is a rational number written as one integer (the numerator) divided by a non-zero integer (the denominator), for example, $\small \frac{1}{3}$ , $\small \frac{3}{4}$ and $\small \frac{4}{3}$ .

A fraction is said to be a proper fraction if the absolute value of the numerator is less than the absolute value of the denominator—that is, if the absolute value of the entire fraction is less than 1 (e.g. $\small \frac{4}{9}$ )—but an improper fraction (US, British or Australian) or top-heavy fraction (British only) if the absolute value of the numerator is greater than or equal to the absolute value of the denominator (e.g. $\small \frac{9}{7}$ ).

Mixed numbers

A mixed number is the sum of a whole number and a proper fraction. For instance, in referring to two entire cakes and three quarters of another cake, the whole and fractional parts of the number are written next to each other: $2+\frac{3}{4}=2\frac{3}{4}$ .

An improper fraction can be thought of as another way to write a mixed number; in the " $2\frac{3}{4}$ " example above, imagine that the two entire cakes are each divided into quarters. Each entire cake contributes $\frac{4}{4}$ to the total, so $\frac{4}{4}+\frac{4}{4}+\frac{3}{4}=\frac{11}{4}$ is another way of writing $2\frac{3}{4}$ .

A mixed number can be converted to an improper fraction in three steps:

Multiply the whole part by the denominator of the fractional part.
Add the numerator of the fractional part to that product.
The resulting sum is the numerator of the new (improper) fraction, and the new denominator is the same as that of the fractional part of the mixed number.

Similarly, an improper fraction can be converted to a mixed number:

Divide the numerator by the denominator.
The quotient (without remainder) becomes the whole part and the remainder becomes the numerator of the fractional part.
The new denominator is the same as that of the original improper fraction.

Equivalent fractions

Multiplying the numerator and denominator of a fraction by the same (non-zero) number results in a new fraction that is said to be equivalent to the original fraction. The word equivalent means that the two fractions have the same value. This is true because for any number n, multiplying by $\frac{n}{n}$ is really multiplying by one, and any number multiplied by one has the same value as the original number. For instance, consider the fraction $\frac{1}{2}$ : when the numerator and denominator are both multiplied by 2, the result is $\frac{2}{4}$ , which has the same value (0.5) as $\frac{1}{2}$ . To picture this visually, imagine cutting the example cake into four pieces; two of the pieces together ( $\frac{2}{4}$ ) make up half the cake ( $\frac{1}{2}$ ).

For example: $\frac{1}{3}$ , $\frac{2}{6}$ , $\frac{3}{9}$ and $\frac{100}{300}$ are all equivalent fractions.

Dividing the numerator and denominator of a fraction by the same non-zero number will also yield an equivalent fraction. this is called reducing or simplifying the fraction. A fraction in which the numerator and denominator have no factors in common (other than 1) is said to be irreducible or in its lowest or simplest terms. For instance, $\frac{3}{9}$ is not in lowest terms because both 3 and 9 can be exactly divided by 3. In contrast, $\frac{3}{8}$ is in lowest terms—the only number that is a factor of both 3 and 8 is 1.

Reciprocals and the "invisible denominator"

The reciprocal of a fraction is another fraction with the numerator and denominator reversed. The reciprocal of $\frac{3}{7}$ , for instance, is $\frac{7}{3}$ .

Because any number divided by 1 results in the same number, it is possible to write any whole number as a fraction by using 1 as the denominator: 17 = $\frac{17}{1}$ (1 is sometimes referred to as the "invisible denominator"). Therefore, except for zero, every fraction or whole number has a reciprocal. The reciprocal of 17 would be $\frac{1}{17}$ .

Complex fractions

A complex fraction (or compound fraction) is a fraction in which the numerator and denominator contain a fraction. For example, $\frac{\frac{1}{2}}{\frac{1}{3}}$ is a complex fraction. To simplify a complex fraction, divide the numerator by the denominator, as with any other fraction: $\frac{\frac{1}{2}}{\frac{1}{3}}=\frac{3}{2}$ .

Arithmetic with fractions

Fractions, like whole numbers, obey the commutative, associative, and distributive laws, and the rule against division by zero.

Comparing fractions

Comparing fractions with the same denominator only requires comparing the numerators.

$\frac{3}{4}>\frac{2}{4}$ as $3>2$ .

In order to compare fractions with different denominators, these are converted to a common denominator: to compare $\frac{a}{b}$ and $\frac{c}{d}$ , these are converted to $\frac{ad}{bd}$ and $\frac{bc}{bd}$ , where bd is the product of the denominators, and then the numerators ad and bc are compared.

$\frac{2}{3}$ ? $\frac{1}{2}$ gives $\frac{4}{6}>\frac{3}{6}$

This method is also known as the "cross-multiply" method which can be explained by multiplying the top and bottom numbers crosswise. The product of the denominators is used as a common (but not necessary the least common) denominator.

$\frac{5}{18}$ ? $\frac{4}{17}$

Multiply 17 by 5 and 18 by 4. Place the products of the equations on top of the denominators. The highest number identifies the largest fraction. Therefore $\frac{5}{18}>\frac{4}{17}$ as 17 × 5 = 85 is greater than 18 × 4 = 72.

In order to work with smaller numbers, the least common denominator is used instead of the product. The fractions are converted to fractions with the least common denominator, and then the numerators are compared.

$\frac{5}{6}$ ? $\frac{3}{4}$ gives $\frac{10}{12}>\frac{9}{12}$

Some standards-based mathematics texts such as Connected Mathematics omit instruction of least common denominators entirely. That text presents the use of "fraction strips" (a strip of paper folded into fractions) or "benchmark fractions" such as one-half against which a fraction such as two-fifths may be compared. While such methods may be useful to build conceptual understanding, they are controversial as they are not effective beyond the elementary school level, and such texts are often supplemented by teachers with the standard method.

Addition

The first rule of addition is that only like quantities can be added; for example, various quantities of quarters. Unlike quantities, such as adding thirds to quarters, must first be converted to like quantities as described below: Imagine a pocket containing two quarters, and another pocket containing three quarters; in total, there are five quarters. Since four quarters is equivalent to one (dollar), this can be represented as follows:

$\frac24+\frac34=\frac54=1\frac14$ .

If $\frac12$ of a cake is to be added to $\frac14$ of a cake, the pieces need to be converted into comparable quantities, such as cake-eighths or cake-quarters.

Adding unlike quantities

To add fractions containing unlike quantities (e.g. quarters and thirds), it is necessary to convert all amounts to like quantities. It is easy to work out the type of fraction to convert to; simply multiply together the two denominators (bottom number) of each fraction.

For adding quarters to thirds, both types of fraction are converted to $\frac14\times\frac13=\frac1{12}$ (twelfths).

Consider adding the following two quantities:

$\frac34+\frac23$

First, convert $\frac34$ into twelfths by multiplying both the numerator and denominator by three: $\frac34\times\frac33=\frac9{12}$ . Note that $\frac33$ is equivalent to 1, which shows that $\frac34$ is equivalent to the resulting $\frac9{12}$

Secondly, convert $\frac23$ into twelfths by multiplying both the numerator and denominator by four: $\frac23\times\frac44=\frac8{12}$ . Note that $\frac44$ is equivalent to 1, which shows that $\frac23$ is equivalent to the resulting $\frac8{12}$

Now it can be seen that:

$\frac34+\frac23$

is equivalent to:

$\frac9{12}+\frac8{12}=\frac{17}{12}=1\frac5{12}$

This method always works, but sometimes there is a smaller denominator that can be used (a least common denominator). For example, to add $\frac{3}{4}$ and $\frac{5}{12}$ the denominator 48 can be used (the product of 4 and 12), but the smaller denominator 12 may also be used, being the least common multiple of 4 and 12.

$\frac34+\frac{5}{12}=\frac{9}{12}+\frac{5}{12}=\frac{14}{12}=\frac76=1\frac16$

Subtraction

The process for subtracting fractions is, in essence, the same as that of adding them: find a common denominator, and change each fraction to an equivalent fraction with the chosen common denominator. The resulting fraction will have that denominator, and its numerator will be the result of subtracting the numerators of the original fractions. For instance,

$\frac23-\frac12=\frac46-\frac36=\frac16$

Multiplication

When multiplying or dividing, it may be possible to choose to cancel down crosswise multiples that share a common factor.

Multiplication by whole numbers

Considering the cake example above, if you have a quarter of the cake and you multiply the amount by three, then you end up with three quarters. We can write this numerically as follows:

$\textstyle{3 \times {1 \over 4} = {3 \over 4}}\,\!$

As another example, suppose that five people work for three hours out of a seven hour day (ie. for three sevenths of the work day). In total, they will have worked for 15 hours (5 x 3 hours each), or 15 sevenths of a day. Since 7 sevenths of a day is a whole day and 14 sevenths is two days, then in total, they will have worked for 2 days and a seventh of a day. Numerically:

$\textstyle{5 \times {3 \over 7} = {15 \over 7} = 2{1 \over 7}} \,\!$

Multiplication by fractions

Considering the cake example above, if you have a quarter of the cake and you multiply the amount by a third, then you end up with a twelfth of the cake. In other words, a third of a quarter (or a third times a quarter) is a twelfth. Why? Because we are splitting each quarter into three pieces, and four quarters times three makes 12 parts (or twelfths). We can write this numerically as follows:

$\textstyle{{1 \over 3} \times {1 \over 4} = {1 \over 12}}\,\!$

As another example, suppose that five people do an equal amount of work that totals three hours out of a seven hour day. Each person will have done a fifth of the work, so they will have worked for a fifth of three sevenths of a day. Numerically:

$\textstyle{{1 \over 5} \times {3 \over 7} = {3 \over 35}}\,\!$

General rule

You may have noticed that when we multiply fractions, we multiply the two numerators (the top numbers) to make the new numerator, and multiply the two denominators (the bottom numbers) to make the new denominator. For example:

$\textstyle{{5 \over 6} \times {7 \over 8} = {5 \times 7 \over 6 \times 8} = {35 \over 48}}\,\!$

Multiplication by mixed numbers

When multiplying mixed numbers, it's best to convert the whole part of the mixed number into a fraction. For example:

$\textstyle{3 \times 2{3 \over 4} = 3 \times \left ({{8 \over 4} + {3 \over 4}} \right ) = 3 \times {11 \over 4} = {33 \over 4} = 8{1 \over 4}}\,\!$

In other words, $\textstyle{2{3 \over 4}}$ is the same as $\textstyle{({8 \over 4} + {3 \over 4})}$ , making 11 quarters in total (because 2 cakes, each split into quarters makes 8 quarters total) and 33 quarters is $\textstyle{8{1 \over 4}}$ , since 8 cakes, each made of quarters, is 32 quarters in total.

Division

To divide by a fraction, simply multiply by the reciprocal of that fraction.

$\textstyle{5 \div {1 \over 2} = 5 \times {2 \over 1} = 5 \times 2 = 10}$

$\textstyle{{2 \over 3} \div {2 \over 5} = {2 \over 3} \times {5 \over 2} = {10 \over 6} = {5 \over 3}}$

To understand why this works, consider the following:

Question, does

$\textstyle{\frac a b \div \frac c d = \frac a b \times \frac d c}$

Given/Accepted

I. Any number divided by itself is one (e.g. $\textstyle{\frac d d = \frac 1 1}$ )

II. When a number is multiplied by one it does not change (e.g. $\textstyle{\frac a b \times \frac 1 1 = \frac a b \times \frac d d = \frac a b}$ )

III. If two fractions have common denominators, then the numerators may be divided to find the quotient (e.g. $\textstyle{\frac {ad}{bd} \div \frac {bc}{bd} = ad \div bc}$ )

Proof

1. $\textstyle{\frac {a} {b} \div \frac {c} {d}}$ , Problem

2. $\textstyle{\frac {a d} {b d} \div \frac {b c} {b d}}$ , Multiplied the first fraction by $\textstyle{\frac d d}$ and the second fraction by $\textstyle{\frac b b}$ , which is the same as multiplying by one, and as accepted above (I & II) does not change the value of the fraction

Note: These values of one were chosen so the fractions would have a common denominator; bd is the common denominator.

3. $\textstyle{\frac {ad}{bd} \div \frac {bc}{bd} = ad \div bc}$ , From what was given in (III)

4. $\textstyle{ad \div bc = \frac {ad}{bc}}$ , Changed notation

5. $\textstyle{\frac {ad}{bc} = \frac a b \times \frac d c }$ , Can be seen

6. $\textstyle{\frac a b \times \frac d c }$ , Solution

About 4,000 years ago Egyptians divided with fractions using slightly different methods, using least common multiples with unit fractions.

Converting repeating decimals to fractions

Decimal numbers, while arguably more useful to work with when performing calculations, lack the same kind of precision that regular fractions (as they are explained in this article) have. Sometimes an infinite number of decimals is required to convey the same kind of precision. Thus, it is often useful to convert repeating decimals into fractions.

For most repeating patterns, a simple division of the pattern by the same number of nines as numbers it has will suffice. For example (the pattern is highlighted in bold):

0.555… = 5/9

0.264264264… = 264/999

0.629162916291… = 6291/9999

In case zeros precede the pattern, the nines are suffixed by the same number of zeros:

0.0555… = 5/90

0.000392392392… = 392/999000

0.00121212… = 12/9900

In case a non-repeating set of decimals precede the pattern (such as 0.1523987987987…), we must equate it as the sum of the non-repeating and repeating parts:

0.1523 + 0.0000987987987…

Then, convert both of these to fractions. Since the first part is not repeating, it is not converted according to the pattern given above:

1523/10000 + 987/9990000

We add these fractions by expressing both with a common divisor...

1521477/9990000 + 987/9990000

And add them.

1522464/9990000

Finally, we simplify it:

31718/208125

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Category: Mathematics

Ratios & Proportions

From Teach And Discover Wiki

Contents

Forms of Fractions

Proper, and Improper Fractions

Mixed numbers

Equivalent fractions

Reciprocals and the "invisible denominator"

Complex fractions

Arithmetic with fractions

Comparing fractions

Addition

Subtraction

Multiplication

Division

Converting repeating decimals to fractions

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