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1.13: Decimal Notation

Difficulty Level: At Grade Created by: CK-12

Which is greater, \begin{align*}\frac{18}{99}\end{align*}1899 or \begin{align*}\frac{15}{80}\end{align*}1580? How can you use decimals to help you with this problem?

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Khan Academy Converting Fractions to Decimals

Guidance

To change a fraction to a decimal number, the numerator must be divided by the denominator. The denominator is the divisor, the numerator is the dividend and the decimal number is the quotient. First change \begin{align*}\frac{3}{4}\end{align*}34 and then change \begin{align*}\frac{3}{13}\end{align*}313 to decimal numbers.

\begin{align*}\frac{3}{4}=3 \div 4\end{align*}34=3÷4

\begin{align*}& \overset{ \ \ {\color{red}0.75}}{4 \overline{ ) {3.00 \;}}}\\ & \underline{-0 \;\;\;\;\;}\\ & \ \ 3 \ 0\\ & \underline{- 2 \ 8 \; \;}\\ & \ \quad \ 20\\ & \underline{- \;\;\; 20 \;}\\ & \qquad \ 0\\\end{align*}4)3.00¯¯¯¯¯¯¯¯¯¯¯¯  0.750  3 02 8  2020 0

When the fraction \begin{align*}\frac{3}{4}\end{align*}34 was divided, the result was a decimal number that ended. There were two digits after the decimal point. The division was complete when the remainder was zero. This decimal number is known as a terminating decimal number.

\begin{align*}\frac{3}{13}=3 \div 13\end{align*}313=3÷13

\begin{align*}& \overset{ \ \ {\color{red}0.2307692307}}{13 \overline{ ) {3.0000000000 \;}}}\\ & \underline{- \ 0 \;}\\ & \quad 3 \ 0\\ & \underline{- \ 2 \ 6 \;}\\ & \qquad 40\\ & \underline{- \;\;\;\;\;39}\\ & \ \qquad \ 10\\ & \underline{- \;\;\;\;\;\;\;\;0}\\ & \ \qquad \ 100\\ & \underline{- \;\;\;\;\;\;\;\;\;91}\\ & \ \qquad \quad \ \ 90\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;78}\\ & \ \qquad \quad \ \ 120\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;117}\\ & \ \qquad \qquad \ \ 30\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;26}\\ & \ \qquad \qquad \quad \ 40\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;39}\\ & \ \qquad \qquad \quad \ \ 10\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;0}\\ & \ \qquad \qquad \quad \ \ 100\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;91}\\ & \ \qquad \qquad \qquad \ \ 9\\\end{align*}13)3.0000000000¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯  0.2307692307 03 0 2 64039  100  10091   9078   120117   3026  4039   100   10091   9

When the fraction \begin{align*}\frac{3}{13}\end{align*}313 was divided, there was no end to the decimal number. However, as the division continued, a pattern developed. This is known as a periodic decimal number with a period length of 6. The period length is the number of digits in the quotient that form a pattern that will repeat itself as the division continues. In the decimal number 0.230 769 230 7... the period is 230 769. If you were to continue dividing, the period would repeat infinitely. The division would never result in a zero remainder.

All periodic decimal numbers can be expressed as common fractions. If the decimal number is non-periodic, it cannot be expressed as a common fraction.

A rational number is any number that be written in the form \begin{align*}\frac{a}{b}\end{align*}ab where \begin{align*}b \ne 0\end{align*}b0. Therefore, periodic decimal numbers and terminating decimal numbers are rational numbers.

Example A

What fraction is equal to 0.45454545?

The decimal number is a periodic or repeating decimal. The period has a length of two. This means that the pattern that is repeating consists of 2 digits. To express the decimal number as a fraction, follow these steps:

Step 1: Let \begin{align*}x=0.45454545\end{align*}x=0.45454545

Step 2: The repeating digit is 45. Place the repeating digit to the left of the decimal point by moving the decimal point 2 places to the right.

45.454545

To move the decimal point two places to the right, the decimal number was multiplied by 100. If you go back to step 1, you will see the equation \begin{align*}x=0.45454545\end{align*}x=0.45454545. If you multiply one side of the equation by 100, you must multiply the other side by 100. You now have \begin{align*}100x=45.454545\end{align*}100x=45.454545

Step 3: The repeating digits must be to the right of the decimal point. The digits are to the right of the decimal point which means that no multiplication is necessary to move the decimal point. The two equations that you now have are \begin{align*}x=0.45454545\end{align*}x=0.45454545 and \begin{align*}100x=45.454545\end{align*}100x=45.454545

Step 4: Subtract the two equations and solve for \begin{align*}x\end{align*}x.

\begin{align*}&100x=45.454545\\ & \underline{\;\; -x=0.45454545}\\ & \frac{99x}{99}=\frac{45}{99}\\ & \quad \ x=\frac{45}{99}=\frac{5}{11}\end{align*}100x=45.454545x=0.4545454599x99=4599 x=4599=511

Example B

Decimal numbers that terminate can be more readily changed to fractions.

What fraction is equal to 0.125?

This decimal appears to be a terminating decimal number. There is no indication that the division continues and no sign of a repeating pattern. The steps to follow to express 0.125 as a fraction are:

Step 1: Express the decimal number as a whole number by moving the decimal point to the right. In this case, the decimal must be moved three places to the right.

Step 2: \begin{align*}0.125=125\end{align*}0.125=125

Express 125 as a fraction with a denominator of 1 and three zeros. The three zeros represent the number of places that the decimal point was moved.

\begin{align*}\frac{125}{1000}\end{align*}1251000

Step 3: If possible, simplify the fraction. If you are not sure of the simplified form, your calculator, TI83, will do the calculations.

Therefore, the decimal number of 0.125 is equivalent to the fraction \begin{align*}\frac{1}{8}\end{align*}18.

The method shown above is one that can be used if you can’t remember the place value associated with the decimal numbers. If you remember the place values, you can simply write the decimal as a fraction and simplify that fraction.

Example C

Are the following decimal numbers terminating or periodic? If they are periodic, what is the period and what is its length?

i) 0.318181818

ii) 0.375

iii) 0.3125

iv) 0.121 122 111 2

Solution:

i) 0.318181818 A periodic decimal number with a period of 18. The length of the period is 2.

ii) 0.375 A terminating decimal number.

iii) 0.3125 A terminating decimal number

iv) 0.121 122 111 2 This decimal number is not a terminating decimal nor is it a periodic decimal. Therefore, the decimal number is not a rational number. Decimal numbers that are non-periodic belong to the irrational numbers.

Concept Problem Revisited

You can convert both fractions to decimals in order to figure out which is greater.

\begin{align*}\frac{18}{99}=.1818...\end{align*}1899=.1818...

\begin{align*}\frac{15}{80}=.1875\end{align*}1580=.1875

You can see that \begin{align*}\frac{15}{80}\end{align*}1580 is greater.

Vocabulary

Irrational Numbers
An irrational number is the set of non-periodic decimal numbers. Some examples of irrational numbers are \begin{align*}\sqrt{3},\sqrt{2}\end{align*}3,2 and \begin{align*}\pi\end{align*}π.
Periodic Decimal
A periodic decimal is a decimal number that has a pattern of digits that repeat. The decimal number 0.146 532 532 5, is a periodic decimal.
Rational Numbers
A rational number is any number that be written in the form \begin{align*}\frac{a}{b}\end{align*}ab where \begin{align*}b \ne 0\end{align*}b0. Therefore, periodic decimal numbers and terminating decimal numbers are rational numbers.
Terminating Decimal
A terminating decimal is a decimal number that ends. The process of dividing the fraction ends when the remainder is zero. The decimal number 0.25 is a terminating decimal.

Guided Practice

1. Express 2.018181818 in the form \begin{align*}\frac{a}{b}\end{align*}ab.

2. Express \begin{align*}\frac{15}{11}\end{align*}1511 in decimal form.

3. If one tablet of micro K contains 0.5 grams of potassium, how much is contained in \begin{align*}2\frac{3}{4}\end{align*}234 tablets?

Answers:

1. 2.018181818

Let \begin{align*}x=2.018181818\end{align*}x=2.018181818 The period is 18.

2018.181818 The period must be placed to the left of the decimal point. This is done by moving the decimal point three places to the right. (Multiply both sides by 1000)

\begin{align*}1000x=2018.181818\end{align*}1000x=2018.181818

2.018181818 The repeating digits must be placed to the right of the decimal point. The decimal point and the repeating digits are separated by a zero. The decimal point must be moved one place to the right. (Multiply both sides by 10)

\begin{align*}10x=20.18181818\end{align*}10x=20.18181818

\begin{align*}& 1000 x=2018.181818 \qquad \text{These are the two equations that must be subtracted.}\\ & \underline{-10x=20.18181818}\\ & \frac{990x}{990}=\frac{1998}{990} \qquad \qquad \quad \ \ \text{Solve for} \ x.\\ & \quad \ \ x=\frac{1998}{990}\end{align*}1000x=2018.181818These are the two equations that must be subtracted.10x=20.18181818990x990=1998990  Solve for x.  x=1998990

Use your calculator to simplify the fraction.

\begin{align*}x=\frac{1998}{990}\end{align*}x=1998990

\begin{align*}x=\frac{111}{55}\end{align*}x=11155

The fraction is in the form of \begin{align*}\frac{a}{b}\end{align*}ab. The decimal number 2.018181818 can be expressed as the fraction \begin{align*}\frac{111}{55}\end{align*}11155.

2. \begin{align*}\frac{15}{11}=15 \div 11\end{align*}1511=15÷11

Long division must be done to express the fraction as a decimal number.

\begin{align*}& \overset{ \ \ 1.363636}{11 \overline{ ) {15.000000 \;}}}\\ & \underline{- \ \ 11 \;}\\ & \qquad 40\\ & \underline{- \;\;\;\;\;33}\\ & \ \qquad \ 70\\ & \underline{- \;\;\;\;\;\;\;66}\\ & \ \qquad \quad 40\\ & \underline{- \;\;\;\;\;\;\;\;\;\;33}\\ & \ \qquad \quad \ \ 70\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;66}\\ & \ \qquad \qquad 40\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;33}\\ & \ \qquad \qquad \ \ 70\\ & \underline{- \;\;\;\;\;\;\;\;\;\;\;\;\;\;\;66}\\ & \ \qquad \qquad \quad \ 4\\\end{align*}11)15.000000¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯  1.363636  114033  7066 4033   7066 4033   7066  4

\begin{align*}\boxed{\frac{15}{11}=1.363636}\end{align*}

3. The number of tablets is given as a mixed number.

\begin{align*}& 2 \frac{3}{4}\\ & \frac{3}{4}=3 \div 4\\ & \overset{ \ \ 0.75}{4 \overline{ ) {3.00 \;}}}\\ & \underline{- 2 \ 8 }\\ & \quad \ \ 20\\ & \underline{- \;\;\;20} \qquad \text{Change the fraction to a decimal number.}\\ & \qquad \ 0\end{align*}

The number of grams of potassium was given as a decimal number. The answer should also be expressed as a decimal number.

The number of tablets is 2.75.

The number of grams of potassium in these tablets is:

\begin{align*}2.75 \times 0.5=1.375 \ grams\end{align*}

Practice

Express the following fractions in decimal form.

  1. \begin{align*}\frac{1}{12}\end{align*}
  2. \begin{align*}\frac{6}{11}\end{align*}
  3. \begin{align*}\frac{3}{20}\end{align*}
  4. \begin{align*}\frac{1}{13}\end{align*}
  5. \begin{align*}\frac{3}{8}\end{align*}

Express the following decimal numbers in the form \begin{align*}\frac{a}{b}\end{align*}.

  1. 0.325
  2. 3.72727272
  3. 0.245454545
  4. 0.618
  5. 0.36363636

Complete the following table.

Problem Fraction Decimal
11. \begin{align*}\frac{5}{64}\end{align*}
12. \begin{align*}\frac{11}{32}\end{align*}
13. \begin{align*}\frac{1}{20}\end{align*}
14. \begin{align*}0.0703125\end{align*}
15. \begin{align*}0.1875\end{align*}

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Difficulty Level:
At Grade
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Date Created:
Dec 19, 2012
Last Modified:
Apr 29, 2014

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