Fraction

A simple fraction (also known as a common fraction or vulgar fraction, where vulgar is Latin for "common") is a rational number written as a/b or ${\displaystyle {\tfrac {a}{b}}}$, where a and b are both integers.[9] As with other fractions, the denominator (b) cannot be zero. Examples include ${\displaystyle {\tfrac {1}{2}}}$, ${\displaystyle -{\tfrac {8}{5}}}$, ${\displaystyle {\tfrac {-8}{5}}}$, and ${\displaystyle {\tfrac {8}{-5}}}$. The term was originally used to distinguish this type of fraction from the sexagesimal fraction used in astronomy.[10]

Common fractions can be positive or negative, and they can be proper or improper (see below). Compound fractions, complex fractions, mixed numerals, and decimals (see below) are not common fractions; though, unless irrational, they can be evaluated to a common fraction.

• A unit fraction is a common fraction with a numerator of 1 (e.g., ${\displaystyle {\tfrac {1}{7}}}$). Unit fractions can also be expressed using negative exponents, as in 2⁻¹, which represents 1/2, and 2⁻², which represents 1/(2²) or 1/4.
• A dyadic fraction is a common fraction in which the denominator is a power of two, e.g. ${\displaystyle {\tfrac {1}{8}}={\tfrac {1}{2^{3}}}}$.

In Unicode, precomposed fraction characters are in the Number Forms block.

Common fractions can be classified as either proper or improper. When the numerator and the denominator are both positive, the fraction is called proper if the numerator is less than the denominator, and improper otherwise.[11][12] The concept of an "improper fraction" is a late development, with the terminology deriving from the fact that "fraction" means "a piece", so a proper fraction must be less than 1.[10] This was explained in the 17th century textbook The Ground of Arts.[13][14]

In general, a common fraction is said to be a proper fraction, if the absolute value of the fraction is strictly less than one—that is, if the fraction is greater than −1 and less than 1.[15][16] It is said to be an improper fraction, or sometimes top-heavy fraction,[17] if the absolute value of the fraction is greater than or equal to 1. Examples of proper fractions are 2/3, −3/4, and 4/9, whereas examples of improper fractions are 9/4, −4/3, and 3/3.

The reciprocal of a fraction is another fraction with the numerator and denominator exchanged. The reciprocal of ${\displaystyle {\tfrac {3}{7}}}$, for instance, is ${\displaystyle {\tfrac {7}{3}}}$. The product of a fraction and its reciprocal is 1, hence the reciprocal is the multiplicative inverse of a fraction. The reciprocal of a proper fraction is improper, and the reciprocal of an improper fraction not equal to 1 (that is, numerator and denominator are not equal) is a proper fraction.

When the numerator and denominator of a fraction are equal (for example, ${\displaystyle {\tfrac {7}{7}}}$), its value is 1, and the fraction therefore is improper. Its reciprocal is identical and hence also equal to 1 and improper.

Any integer can be written as a fraction with the number one as denominator. For example, 17 can be written as ${\displaystyle {\tfrac {17}{1}}}$, where 1 is sometimes referred to as the invisible denominator. Therefore, every fraction or integer, except for zero, has a reciprocal. For example. the reciprocal of 17 is ${\displaystyle {\tfrac {1}{17}}}$.

A ratio is a relationship between two or more numbers that can be sometimes expressed as a fraction. Typically, a number of items are grouped and compared in a ratio, specifying numerically the relationship between each group. Ratios are expressed as "group 1 to group 2 ... to group n". For example, if a car lot had 12 vehicles, of which

• 2 are white,
• 6 are red, and
• 4 are yellow,

then the ratio of red to white to yellow cars is 6 to 2 to 4. The ratio of yellow cars to white cars is 4 to 2 and may be expressed as 4:2 or 2:1.

A ratio is often converted to a fraction when it is expressed as a ratio to the whole. In the above example, the ratio of yellow cars to all the cars on the lot is 4:12 or 1:3. We can convert these ratios to a fraction, and say that 4/12 of the cars or 1/3 of the cars in the lot are yellow. Therefore, if a person randomly chose one car on the lot, then there is a one in three chance or probability that it would be yellow.

A decimal fraction is a fraction whose denominator is not given explicitly, but is understood to be an integer power of ten. Decimal fractions are commonly expressed using decimal notation in which the implied denominator is determined by the number of digits to the right of a decimal separator, the appearance of which (e.g., a period, an interpunct (·), a comma) depends on the locale (for examples, see decimal separator). Thus, for 0.75 the numerator is 75 and the implied denominator is 10 to the second power, namely, 100, because there are two digits to the right of the decimal separator. In decimal numbers greater than 1 (such as 3.75), the fractional part of the number is expressed by the digits to the right of the decimal (with a value of 0.75 in this case). 3.75 can be written either as an improper fraction, 375/100, or as a mixed number, ${\displaystyle 3{\tfrac {75}{100}}}$.

Decimal fractions can also be expressed using scientific notation with negative exponents, such as 6.023×10⁻⁷, which represents 0.0000006023. The 10⁻⁷ represents a denominator of 10⁷. Dividing by 10⁷ moves the decimal point 7 places to the left.

Decimal fractions with infinitely many digits to the right of the decimal separator represent an infinite series. For example, 1/3 = 0.333... represents the infinite series 3/10 + 3/100 + 3/1000 + ....

Another kind of fraction is the percentage (from Latin: per centum, meaning "per hundred", represented by the symbol %), in which the implied denominator is always 100. Thus, 51% means 51/100. Percentages greater than 100 or less than zero are treated in the same way, e.g. 311% equals 311/100, and −27% equals −27/100.

The related concept of permille or parts per thousand (ppt) has an implied denominator of 1000, while the more general parts-per notation, as in 75 parts per million (ppm), means that the proportion is 75/1,000,000.

Whether common fractions or decimal fractions are used is often a matter of taste and context. Common fractions are used most often when the denominator is relatively small. By mental calculation, it is easier to multiply 16 by 3/16 than to do the same calculation using the fraction's decimal equivalent (0.1875). And it is more accurate to multiply 15 by 1/3, for example, than it is to multiply 15 by any decimal approximation of one third. Monetary values are commonly expressed as decimal fractions with denominator 100, i.e., with two decimals, for example $3.75. However, as noted above, in pre-decimal British currency, shillings and pence were often given the form (but not the meaning) of a fraction, as, for example, "3/6" (read "three and six") meaning 3 shillings and 6 pence, and having no relationship to the fraction 3/6.

A mixed numeral (also called a mixed fraction or mixed number) is a traditional denotation of the sum of a non-zero integer and a proper fraction (having the same sign). It is used primarily in measurement: ${\displaystyle 2{\tfrac {3}{16}}}$inches, for example. Scientific measurements almost invariably use decimal notation rather than mixed numbers. The sum can be implied without the use of a visible operator such as the appropriate "+". For example, in referring to two entire cakes and three quarters of another cake, the numerals denoting the integer part and the fractional part of the cakes can be written next to each other as ${\displaystyle 2{\tfrac {3}{4}}}$instead of the unambiguous notation ${\displaystyle 2+{\tfrac {3}{4}}.}$ Negative mixed numerals, as in ${\displaystyle -2{\tfrac {3}{4}}}$, are treated like ${\displaystyle \scriptstyle -\left(2+{\frac {3}{4}}\right).}$ Any such sum of a whole plus a part can be converted to an improper fraction by applying the rules of adding unlike quantities.

This tradition is, formally, in conflict with the notation in algebra where adjacent symbols, without an explicit infix operator, denote a product. In the expression ${\displaystyle 2x}$, the "understood" operation is multiplication. If x is replaced by, for example, the fraction ${\displaystyle {\tfrac {3}{4}}}$, the "understood" multiplication needs to be replaced by explicit multiplication, to avoid the appearance of a mixed number.

When multiplication is intended, ${\displaystyle 2{\tfrac {b}{c}}}$ may be written as

${\displaystyle 2\cdot {\frac {b}{c}},\quad }$ or ${\displaystyle \quad 2\times {\frac {b}{c}},\quad }$ or ${\displaystyle \quad 2\left({\frac {b}{c}}\right),\;\ldots }$

An improper fraction can be converted to a mixed number as follows:

1. Using Euclidean division (division with remainder), divide the numerator by the denominator. In the example, ${\displaystyle {\tfrac {11}{4}}}$, divide 11 by 4. 11 ÷ 4 = 2 remainder 3.
2. The quotient (without the remainder) becomes the whole number part of the mixed number. The remainder becomes the numerator of the fractional part. In the example, 2 is the whole number part and 3 is the numerator of the fractional part.
3. The new denominator is the same as the denominator of the improper fraction. In the example, it is 4. Thus, ${\displaystyle {\tfrac {11}{4}}=2{\tfrac {3}{4}}}$.

An Egyptian fraction is the sum of distinct positive unit fractions, for example ${\displaystyle {\tfrac {1}{2}}+{\tfrac {1}{3}}}$. This definition derives from the fact that the ancient Egyptians expressed all fractions except ${\displaystyle {\tfrac {1}{2}}}$, ${\displaystyle {\tfrac {2}{3}}}$ and ${\displaystyle {\tfrac {3}{4}}}$ in this manner. Every positive rational number can be expanded as an Egyptian fraction. For example, ${\displaystyle {\tfrac {5}{7}}}$ can be written as ${\displaystyle {\tfrac {1}{2}}+{\tfrac {1}{6}}+{\tfrac {1}{21}}.}$ Any positive rational number can be written as a sum of unit fractions in infinitely many ways. Two ways to write ${\displaystyle {\tfrac {13}{17}}}$ are ${\displaystyle {\tfrac {1}{2}}+{\tfrac {1}{4}}+{\tfrac {1}{68}}}$ and ${\displaystyle {\tfrac {1}{3}}+{\tfrac {1}{4}}+{\tfrac {1}{6}}+{\tfrac {1}{68}}}$.

In a complex fraction, either the numerator, or the denominator, or both, is a fraction or a mixed number,[18][19] corresponding to division of fractions. For example, ${\displaystyle {\frac {\tfrac {1}{2}}{\tfrac {1}{3}}}}$ and ${\displaystyle {\frac {12{\tfrac {3}{4}}}{26}}}$ are complex fractions. To reduce a complex fraction to a simple fraction, treat the longest fraction line as representing division. For example:

${\displaystyle {\frac {\tfrac {1}{2}}{\tfrac {1}{3}}}={\tfrac {1}{2}}\times {\tfrac {3}{1}}={\tfrac {3}{2}}}$

${\displaystyle {\frac {12{\tfrac {3}{4}}}{26}}=12{\tfrac {3}{4}}\cdot {\tfrac {1}{26}}={\tfrac {12\cdot 4+3}{4}}\cdot {\tfrac {1}{26}}={\tfrac {51}{4}}\cdot {\tfrac {1}{26}}={\tfrac {51}{104}}}$

${\displaystyle {\frac {\tfrac {3}{2}}{5}}={\tfrac {3}{2}}\times {\tfrac {1}{5}}={\tfrac {3}{10}}}$

${\displaystyle {\frac {8}{\tfrac {1}{3}}}=8\times {\tfrac {3}{1}}=24.}$

If, in a complex fraction, there is no unique way to tell which fraction lines takes precedence, then this expression is improperly formed, because of ambiguity. So 5/10/20/40 is not a valid mathematical expression, because of multiple possible interpretations, e.g. as

${\displaystyle 5/(10/(20/40))={\frac {5}{10/{\tfrac {20}{40}}}}={\frac {1}{4}}\quad }$ or as ${\displaystyle \quad (5/10)/(20/40)={\frac {\tfrac {5}{10}}{\tfrac {20}{40}}}=1}$

A compound fraction is a fraction of a fraction, or any number of fractions connected with the word of,[18][19] corresponding to multiplication of fractions. To reduce a compound fraction to a simple fraction, just carry out the multiplication (see the section on multiplication). For example, ${\displaystyle {\tfrac {3}{4}}}$ of ${\displaystyle {\tfrac {5}{7}}}$ is a compound fraction, corresponding to ${\displaystyle {\tfrac {3}{4}}\times {\tfrac {5}{7}}={\tfrac {15}{28}}}$. The terms compound fraction and complex fraction are closely related and sometimes one is used as a synonym for the other. (For example, the compound fraction ${\displaystyle {\tfrac {3}{4}}\times {\tfrac {5}{7}}}$ is equivalent to the complex fraction ${\displaystyle {\tfrac {3/4}{7/5}}}$.)

Nevertheless, "complex fraction" and "compound fraction" may both be considered outdated[20] and now used in no well-defined manner, partly even taken synonymously for each other[21] or for mixed numerals.[22] They have lost their meaning as technical terms and the attributes "complex" and "compound" tend to be used in their every day meaning of "consisting of parts".

Multiplying the numerator and denominator of a fraction by the same (non-zero) number results in a fraction that is equivalent to the original fraction. This is true because for any non-zero number ${\displaystyle n}$, the fraction ${\displaystyle {\tfrac {n}{n}}}$ equals ${\displaystyle 1}$. Therefore, multiplying by ${\displaystyle {\tfrac {n}{n}}}$ is the same as multiplying by one, and any number multiplied by one has the same value as the original number. By way of an example, start with the fraction ${\displaystyle {\tfrac {1}{2}}}$. When the numerator and denominator are both multiplied by 2, the result is ${\displaystyle {\tfrac {2}{4}}}$, which has the same value (0.5) as ${\displaystyle {\tfrac {1}{2}}}$. To picture this visually, imagine cutting a cake into four pieces; two of the pieces together (${\displaystyle {\tfrac {2}{4}}}$) make up half the cake (${\displaystyle {\tfrac {1}{2}}}$).

Dividing the numerator and denominator of a fraction by the same non-zero number yields an equivalent fraction: if the numerator and the denominator of a fraction are both divisible by a number (called a factor) greater than 1, then the fraction can be reduced to an equivalent fraction with a smaller numerator and a smaller denominator. For example, if both the numerator and the denominator of the fraction ${\displaystyle {\tfrac {a}{b}}}$ are divisible by ${\displaystyle c,}$ then they can be written as ${\displaystyle a=cd}$ and ${\displaystyle b=ce,}$ and the fraction becomes ${\displaystyle {\tfrac {cd}{ce}}}$, which can be reduced by dividing both the numerator and denominator by ${\displaystyle c}$ to give the reduced fraction ${\displaystyle {\tfrac {d}{e}}.}$

If one takes for c the greatest common divisor of the numerator and the denominator, one gets the equivalent fraction whose numerator and denominator have the lowest absolute values. One says that the fraction has been reduced to its lowest terms.

If the numerator and the denominator do not share any factor greater than 1, the fraction is already reduced to its lowest terms, and it is said to be irreducible, reduced, or in simplest terms. For example, ${\displaystyle {\tfrac {3}{9}}}$ is not in lowest terms because both 3 and 9 can be exactly divided by 3. In contrast, ${\displaystyle {\tfrac {3}{8}}}$ is in lowest terms—the only positive integer that goes into both 3 and 8 evenly is 1.

Using these rules, we can show that ${\displaystyle {\tfrac {5}{10}}={\tfrac {1}{2}}={\tfrac {10}{20}}={\tfrac {50}{100}}}$, for example.

As another example, since the greatest common divisor of 63 and 462 is 21, the fraction ${\displaystyle {\tfrac {63}{462}}}$ can be reduced to lowest terms by dividing the numerator and denominator by 21:

${\displaystyle {\tfrac {63}{462}}={\tfrac {63\,\div \,21}{462\,\div \,21}}={\tfrac {3}{22}}}$

The Euclidean algorithm gives a method for finding the greatest common divisor of any two integers.

Comparing fractions with the same positive denominator yields the same result as comparing the numerators:

${\displaystyle {\tfrac {3}{4}}>{\tfrac {2}{4}}}$ because 3 > 2, and the equal denominators ${\displaystyle 4}$ are positive.

If the equal denominators are negative, then the opposite result of comparing the numerators holds for the fractions:

${\displaystyle {\tfrac {3}{-4}}<{\tfrac {2}{-4}}{\text{ because }}{\tfrac {a}{-b}}={\tfrac {-a}{b}}{\text{ and }}-3<-2.}$

If two positive fractions have the same numerator, then the fraction with the smaller denominator is the larger number. When a whole is divided into equal pieces, if fewer equal pieces are needed to make up the whole, then each piece must be larger. When two positive fractions have the same numerator, they represent the same number of parts, but in the fraction with the smaller denominator, the parts are larger.

One way to compare fractions with different numerators and denominators is to find a common denominator. To compare ${\displaystyle {\tfrac {a}{b}}}$ and ${\displaystyle {\tfrac {c}{d}}}$, these are converted to ${\displaystyle {\tfrac {a\cdot d}{b\cdot d}}}$ and ${\displaystyle {\tfrac {b\cdot c}{b\cdot d}}}$ (where the dot signifies multiplication and is an alternative symbol to ×). Then bd is a common denominator and the numerators ad and bc can be compared. It is not necessary to determine the value of the common denominator to compare fractions – one can just compare ad and bc, without evaluating bd, e.g., comparing ${\displaystyle {\tfrac {2}{3}}}$ ? ${\displaystyle {\tfrac {1}{2}}}$ gives ${\displaystyle {\tfrac {4}{6}}>{\tfrac {3}{6}}}$.

For the more laborious question ${\displaystyle {\tfrac {5}{18}}}$ ? ${\displaystyle {\tfrac {4}{17}},}$ multiply top and bottom of each fraction by the denominator of the other fraction, to get a common denominator, yielding ${\displaystyle {\tfrac {5\times 17}{18\times 17}}}$ ? ${\displaystyle {\tfrac {18\times 4}{18\times 17}}}$. It is not necessary to calculate ${\displaystyle 18\times 17}$ – only the numerators need to be compared. Since 5×17 (= 85) is greater than 4×18 (= 72), the result of comparing is ${\displaystyle {\tfrac {5}{18}}>{\tfrac {4}{17}}}$.

Because every negative number, including negative fractions, is less than zero, and every positive number, including positive fractions, is greater than zero, it follows that any negative fraction is less than any positive fraction. This allows, together with the above rules, to compare all possible fractions.

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:

${\displaystyle {\tfrac {2}{4}}+{\tfrac {3}{4}}={\tfrac {5}{4}}=1{\tfrac {1}{4}}}$.

If ${\displaystyle {\tfrac {1}{2}}}$ of a cake is to be added to ${\displaystyle {\tfrac {1}{4}}}$ of a cake, the pieces need to be converted into comparable quantities, such as cake-eighths or cake-quarters.

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 chosen type of fraction to convert to; simply multiply together the two denominators (bottom number) of each fraction. In case of an integer number apply the invisible denominator ${\displaystyle 1.}$

For adding quarters to thirds, both types of fraction are converted to twelfths, thus:

${\displaystyle {\frac {1}{4}}\ +{\frac {1}{3}}={\frac {1\times 3}{4\times 3}}\ +{\frac {1\times 4}{3\times 4}}={\frac {3}{12}}\ +{\frac {4}{12}}={\frac {7}{12}}.}$

Consider adding the following two quantities:

${\displaystyle {\frac {3}{5}}+{\frac {2}{3}}}$

First, convert ${\displaystyle {\tfrac {3}{5}}}$ into fifteenths by multiplying both the numerator and denominator by three: ${\displaystyle {\tfrac {3}{5}}\times {\tfrac {3}{3}}={\tfrac {9}{15}}}$. Since ${\displaystyle {\tfrac {3}{3}}}$ equals 1, multiplication by ${\displaystyle {\tfrac {3}{3}}}$ does not change the value of the fraction.

Second, convert ${\displaystyle {\tfrac {2}{3}}}$ into fifteenths by multiplying both the numerator and denominator by five: ${\displaystyle {\tfrac {2}{3}}\times {\tfrac {5}{5}}={\tfrac {10}{15}}}$.

Now it can be seen that:

${\displaystyle {\frac {3}{5}}+{\frac {2}{3}}}$

is equivalent to:

${\displaystyle {\frac {9}{15}}+{\frac {10}{15}}={\frac {19}{15}}=1{\frac {4}{15}}}$

This method can be expressed algebraically:

${\displaystyle {\frac {a}{b}}+{\frac {c}{d}}={\frac {ad+cb}{bd}}}$

This algebraic method always works, thereby guaranteeing that the sum of simple fractions is always again a simple fraction. However, if the single denominators contain a common factor, a smaller denominator than the product of these can be used. For example, when adding ${\displaystyle {\tfrac {3}{4}}}$ and ${\displaystyle {\tfrac {5}{6}}}$ the single denominators have a common factor ${\displaystyle 2,}$ and therefore, instead of the denominator 24 (4 × 6), the halved denominator 12 may be used, not only reducing the denominator in the result, but also the factors in the numerator.

${\displaystyle {\begin{aligned}{\frac {3}{4}}+{\frac {5}{6}}&={\frac {3\cdot 6}{4\cdot 6}}+{\frac {4\cdot 5}{4\cdot 6}}={\frac {18}{24}}+{\frac {20}{24}}&={\frac {19}{12}}\\&={\frac {3\cdot 3}{4\cdot 3}}+{\frac {2\cdot 5}{2\cdot 6}}={\frac {9}{12}}+{\frac {10}{12}}&={\frac {19}{12}}\end{aligned}}}$

The smallest possible denominator is given by the least common multiple of the single denominators, which results from dividing the rote multiple by all common factors of the single denominators. This is called the least common denominator.

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,

${\displaystyle {\tfrac {2}{3}}-{\tfrac {1}{2}}={\tfrac {4}{6}}-{\tfrac {3}{6}}={\tfrac {1}{6}}}$

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

${\displaystyle {\frac {2}{3}}\times {\frac {3}{4}}={\frac {6}{12}}}$

To explain the process, consider one third of one quarter. Using the example of a cake, if three small slices of equal size make up a quarter, and four quarters make up a whole, twelve of these small, equal slices make up a whole. Therefore, a third of a quarter is a twelfth. Now consider the numerators. The first fraction, two thirds, is twice as large as one third. Since one third of a quarter is one twelfth, two thirds of a quarter is two twelfth. The second fraction, three quarters, is three times as large as one quarter, so two thirds of three quarters is three times as large as two thirds of one quarter. Thus two thirds times three quarters is six twelfths.

A short cut for multiplying fractions is called "cancellation". Effectively the answer is reduced to lowest terms during multiplication. For example:

${\displaystyle {\frac {2}{3}}\times {\frac {3}{4}}={\frac {{\cancel {2}}^{~1}}{{\cancel {3}}^{~1}}}\times {\frac {{\cancel {3}}^{~1}}{{\cancel {4}}^{~2}}}={\frac {1}{1}}\times {\frac {1}{2}}={\frac {1}{2}}}$

A two is a common factor in both the numerator of the left fraction and the denominator of the right and is divided out of both. Three is a common factor of the left denominator and right numerator and is divided out of both.

Since a whole number can be rewritten as itself divided by 1, normal fraction multiplication rules can still apply.

${\displaystyle 6\times {\tfrac {3}{4}}={\tfrac {6}{1}}\times {\tfrac {3}{4}}={\tfrac {18}{4}}}$

This method works because the fraction 6/1 means six equal parts, each one of which is a whole.

When multiplying mixed numbers, it is considered preferable to convert the mixed number into an improper fraction.[23] For example:

${\displaystyle 3\times 2{\frac {3}{4}}=3\times \left({\frac {8}{4}}+{\frac {3}{4}}\right)=3\times {\frac {11}{4}}={\frac {33}{4}}=8{\frac {1}{4}}}$

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

To divide a fraction by a whole number, you may either divide the numerator by the number, if it goes evenly into the numerator, or multiply the denominator by the number. For example, ${\displaystyle {\tfrac {10}{3}}\div 5}$ equals ${\displaystyle {\tfrac {2}{3}}}$ and also equals ${\displaystyle {\tfrac {10}{3\cdot 5}}={\tfrac {10}{15}}}$, which reduces to ${\displaystyle {\tfrac {2}{3}}}$. To divide a number by a fraction, multiply that number by the reciprocal of that fraction. Thus, ${\displaystyle {\tfrac {1}{2}}\div {\tfrac {3}{4}}={\tfrac {1}{2}}\times {\tfrac {4}{3}}={\tfrac {1\cdot 4}{2\cdot 3}}={\tfrac {2}{3}}}$.

To change a common fraction to a decimal, do a long division of the decimal representations of the numerator by the denominator (this is idiomatically also phrased as "divide the denominator into the numerator"), and round the answer to the desired accuracy. For example, to change 1/4 to a decimal, divide 1.00 by 4 ("4 into 1.00"), to obtain 0.25. To change 1/3 to a decimal, divide 1.000... by 3 ("3 into 1.000..."), and stop when the desired accuracy is obtained, e.g., at 4 decimals with 0.3333. The fraction 1/4 can be written exactly with two decimal digits, while the fraction 1/3 cannot be written exactly as a decimal with a finite number of digits. To change a decimal to a fraction, write in the denominator a 1 followed by as many zeroes as there are digits to the right of the decimal point, and write in the numerator all the digits of the original decimal, just omitting the decimal point. Thus ${\displaystyle 12.3456={\tfrac {123456}{10000}}.}$

Decimal numbers, while arguably more useful to work with when performing calculations, sometimes lack the precision that common fractions have. Sometimes an infinite repeating decimal is required to reach the same precision. Thus, it is often useful to convert repeating decimals into fractions.

A conventional way to indicate a repeating decimal is to place a bar (known as a vinculum) over the digits that repeat, for example 0.789 = 0.789789789... For repeating patterns that begin immediately after the decimal point, the result of the conversion is the fraction with the pattern as a numerator, and the same number of nines as a denominator. For example:

0.5 = 5/9

0.62 = 62/99

0.264 = 264/999

0.6291 = 6291/9999

If leading zeros precede the pattern, the nines are suffixed by the same number of trailing zeros:

0.05 = 5/90

0.000392 = 392/999000

0.0012 = 12/9900

If a non-repeating set of decimals precede the pattern (such as 0.1523987), one may write the number as the sum of the non-repeating and repeating parts, respectively:

0.1523 + 0.0000987

Then, convert both parts to fractions, and add them using the methods described above:

1523 / 10000 + 987 / 9990000 = 1522464 / 9990000

Alternatively, algebra can be used, such as below:

1. Let x = the repeating decimal:

   x = 0.1523987

2. Multiply both sides by the power of 10 just great enough (in this case 10⁴) to move the decimal point just before the repeating part of the decimal number:

   10,000x = 1,523.987

3. Multiply both sides by the power of 10 (in this case 10³) that is the same as the number of places that repeat:

   10,000,000x = 1,523,987.987

4. Subtract the two equations from each other (if a = b and c = d, then a − c = b − d):

   10,000,000x − 10,000x = 1,523,987.987 − 1,523.987

5. Continue the subtraction operation to clear the repeating decimal:

   9,990,000x = 1,523,987 − 1,523

   9,990,000x = 1,522,464

6. Divide both sides by 9,990,000 to represent x as a fraction

   x = 1522464/9990000

In primary schools, fractions have been demonstrated through Cuisenaire rods, Fraction Bars, fraction strips, fraction circles, paper (for folding or cutting), pattern blocks, pie-shaped pieces, plastic rectangles, grid paper, dot paper, geoboards, counters and computer software.

Several states in the United States have adopted learning trajectories from the Common Core State Standards Initiative's guidelines for mathematics education. Aside from sequencing the learning of fractions and operations with fractions, the document provides the following definition of a fraction: "A number expressible in the form ${\displaystyle a}$/${\displaystyle b}$ where ${\displaystyle a}$ is a whole number and ${\displaystyle b}$ is a positive whole number. (The word fraction in these standards always refers to a non-negative number.)"[38] The document itself also refers to negative fractions.