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Lesson 20 INTRODUCTION TO FRACTIONSCOMMON FRACTIONS
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What you see above is called a number line. On it will be the numbers we need for measuring -- which is very different from counting. We count with the names of the natural numbers --
1 person, 2 persons, 3
persons, and so on -- and each 1 is indivisible. You cannot take half
of 1 person. But when we measure a length, for example, it will not
always be a whole number, because length is continuous. It could have any part -- halves, thirds, fourths, fifths, millionths
In order to measure, then, we must have numbers that are parts of number 1. That requires a new and very different kind of number. What we call a proper fraction is a number that is less than 1.
Number 1 is now a continuous unit.
In this Lesson, we will answer the following:
| 1. | What is a common fraction? |
| A number written with a numerator and a denominator, in which both are natural numbers. | |
| Example 1. | 3 10 |
("three-tenths") is a common fraction. It is in contrast |
to .3, which is a decimal fraction, or simply a decimal.
| 2. | What do the denominator and the numerator signify? |
| The denominator names the number of equal parts into which number 1 has been divided. The numerator names how many of those parts to count. |
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| The fraction | 3 10 |
signifies that number 1 has been divided into 10 |
equal parts and -- starting from the left -- we are counting 3 of them.
Number 1, in other words, has been divided into tenths. At this point, the student should be clear about the language of division into equal parts, and why we use ordinal numbers: third, fourth, fifth, sixth, and so on. See Lesson 3, the topic Division into equal parts.
Since the numerator and denominator are natural numbers, the numerator has a ratio to the denominator. (3 is three tenths of 10.) And the fraction itself has that same ratio to 1. Fractions therefore are also called rational numbers.
(We speak throughout of number 1, yet it assumes the role of any unit of measure. 1 centimeter, 1 inch, 1 pound, 1 hour. Again, fractions are numbers we need for measuring.)
In everyday speech, a fraction means a part (a third, a fourth, a fifth) of a whole, as in the phrase "a fraction of the students." But students are discrete not continuous, and in mathematics a fraction is a number we need to measure what is continuous. A mathematical fraction is not simply a part of any whole. It is a part of the unit of measure, which is 1. As for a pie divided into equal parts, it can be and should be described verbally -- not with fractions. We do not measure pies. We will go into this more below.
Example 2. If number 1 is divided into 5 equal parts, and we count 4 of them, what fraction is that? Also, into which parts has number 1 been divided?
| Answer. That fraction is | 4 5 |
("Four-fifths"). |
Number 1 has been divided into fifths.
Note: To divide number 1 into fifths, we cut the line four times. We cut the line one less than the name of the part.
In the next Lesson we will see that number 1 has been decomposed
into unit fractions, which are fractions whose numerator is 1.
Every fraction is thus a number of unit fractions. Addition and
subtraction, whether with whole numbers or fractions, are done
with a number of units.
| 3. | What is a proper fraction? | |
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| A fraction that is less than 1. | ||
| 4. | How can we recognize a proper fraction? | |
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| The numerator is smaller than the denominator. | ||
| On the number line, it falls to the left of 1. Specifically, | 2 3 |
is two thirds |
of 1, as we will see presently.
| 5. | In English, how are the proper fractions named? | |
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| Since the numerator and denominator are natural numbers, they have a ratio to one another. And a proper fraction has the same name as that ratio. | ||
| Example 3. The number we write as 1 over 2 -- | 1 2 |
-- is called |
"one-half" because of the ratio of 1 to 2. 1 is one half of 2. And the
fraction itself is one half of 1.
We write the name of a fraction hyphenated, but the name of a part, or a ratio, unhyphenated. We explain why below.
| Example 4. Why is this number | 2 3 |
called "two-thirds"? |
Answer. Because of the ratio of 2 to 3. 2 is two thirds of 3. And on the number line, that fraction is two thirds of 1.
| To place | 2 3 |
on the number line, we must cut the length representing 1 |
into thirds, that is, into three equal parts. We cut it only twice.
"One third, two thirds."
Example 5. This number Πis called "one-quarter" or "one-fourth," because the numerator is one quarter of the denominator -- and Πitself is one quarter of 1.
To place Πon the number line, cut the line one less than the name of the part. To divide 1 into fourths, cut the line three times.
Example 6. Read these numbers:
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"Five-eighths" | |
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"Sixteen-thirds" | The improper fractions (see below) are named like the proper fractions. |
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"Nine-halves" | (Not "Nine 2's.") |
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"27 over 39" | With large numbers, it is not necessary to say "27 thirty-ninths." |
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"56 thousandths" | When the denominator is a power of 10, however, we always say the decimal name. (Lesson 3) |
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"5 and 79 hundredths" | This is a mixed number; see below. |
Example 7. What number is at the arrow?
Answer. Number 1 has been cut five times -- into six equal parts. That
| number is | 5 6 |
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The proper fractions are the parts of 1.
| 6. | What is a mixed number? |
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| A whole number plus a proper fraction. | |
| Example 8. 2 | 1 3 |
-- "2 and one-third" -- is a mixed number. It is 2 plus | 1 3 |
. |
The and in "2 and one-third" means plus.
| To place 2 | 1 3 |
on the number line, the unit between 2 and 3 must be cut |
into thirds. We cut the line twice.
Improper fractions
If we divide each whole unit into thirds, say, and keep counting them --
| -- then we will come to | 3 3 |
, | 4 3 |
, | 5 3 |
, and so on. That is, we will come to |
fractions that are equal to or greater than 1. We call those improper fractions.
| 7. | What is an improper fraction? |
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| A fraction greater than or equal to 1. |
We can recognize an improper fraction when the numerator is greater than or equal to the denominator.
In fact, when the numerator is equal to the denominator,
then the fraction is equal to 1.
| 5 5 |
= | 6 6 |
= | 7 7 |
= | 8 8 |
= 1. |
Those fractions also are improper.
We will see in the next Lesson, Example 4, that we can write 1 equal to a fraction with any denominator.
Problem 1. Which of these fractions are less than 1, equal to 1, or greater than 1?
| 2 3 |
, | 3 2 |
, | 8 5 |
, | 8 8 |
, | 8 9 |
, | 9 9 |
, | 10 9 |
. |
To see the answer, pass your mouse over the colored area.
To cover the answer again, click "Refresh" ("Reload").
Do the problem yourself first!
| Less than 1: | 2 3 |
8 9 |
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| Equal to 1: | 8 8 |
9 9 |
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| Greater than 1: | 3 2 |
8 5 |
10 9 |
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We will now see explicitly why we use the division bar to signify a fraction.
| 8. | How do we change an improper fraction to a mixed number or a whole number? |
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| Divide the numerator by the denominator. Write the quotient (4), and write the remainder (1) as the numerator of the fraction; do not change the denominator. | |
When we change an improper fraction to a mixed number, we say that we are extracting, or taking out, the whole number..
| Example 9. Extract the whole number from | 53 8 |
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| Solution. | 53 8 |
= 6 | 5 8 |
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"8 goes into 53 six (6) times (48) with 5 left over."
We have extracted the whole number 6.
As for the remainder, it is the number we must add to 48 to get 53. Compare Lesson 11
(Obviously, the student must know the multiplication table.)
| Example 10. | 31 9 |
= 3 | 4 9 |
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"9 goes into 31 three (3) times (27) with remainder 4."
27 plus 4 is 31.
Again: The remainder is what we must add to 27 to get 31.
| Example 11. | 28 4 |
= 7 |
"4 goes into 28 seven (7) times exactly."
Problem 2. Answer with a mixed number or with a whole number and a remainder, whichever makes sense.
a) How many basketball teams -- 5 on a team -- could you make from 23
a) students?
23 ÷ 5 = 4 R 3.
You could make 4 teams. 3 students will be left out.
| (4 | 3 5 |
teams makes no sense.) |
b) You are going on a trip of 23 miles, and you have gone a fifth of the
b) distance. How far have you gone?
| 4 | 3 5 |
miles. |
To calculate the fifth part of a number, that is, to divde it into five equal parts, divide by 5. (Lesson 15, Question 4)
| 23 5 |
= 4 | 3 5 |
. |
1 mile, which is continuous, will have any part.
Problem 3.
a) How much is a third of 24 people?
24 ÷ 3 = 8 people.
b) How much is a third of 25 people?
25 people do not have a third part; 3 is not a divisor of the natural number 25. (Lesson 15.)
c) How much is a third of 25 seconds?
| 25 seconds ÷ 3 = 8 | 1 3 |
seconds. |
Time is continuous. It will have any part.
| 9. | How do we change a mixed number to an improper fraction? |
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Multiply the whole number (4) by the denominator (2), and add the numerator (1). Write that sum (9) as the numerator of the improper fraction. Keep the same denominator. |
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For an explanation of why we do that, see Lesson 21, Question 3.
| Examples. 2 | 3 5 |
= | 13 5 |
"5 times 2 is 10, plus 3 is 13; over 5."
| 3 | 5 8 |
= | 29 8 |
"8 times 3 is 24, plus 5 is 29; over 8."
To summarize: Fractions that are less than 1 are called proper fractions, while fractions greater than or equal to 1 are improper. Improper fractions are equivalent to mixed numbers or whole numbers.
In English, the names of the proper fractions are the same as the names of the parts,
and therefore a fraction and a ratio have become confused. "One
quarter" is the name of a part, or a ratio, which is a relationship
between numbers. 5 people are one quarter of 20 people -- and that
statement is not a measurement, we are not invoking any fraction. The
fraction we call "one-quarter," on the other hand, is one quarter of a
continuous 1. That may sound like double-talk, but how else are we to
explain the meaning of the fraction Πand its place on the number
line? Obviously, one must first understand the ratio -- the part -- whose name is one quarter
(To write "5 is Πof 20," or "She ate Πof the pie," is not just stylistically crude. It shows a profound disregard for the distinction between a fraction and the name of a ratio or part.)
Spanish is much more scrupulous in distinguishing the names of the fractions from the names of the ratios. This fraction 
, for example, is called un doceavo, while the ratio of 1 to 12 is la duodécima parte.
We
will respect the separation of ratio and number by writing the name of
the number hyphenated -- three-fourths -- but the name of the ratio
unhyphenated: three fourths. That separation gives life to the
question, Can "the ratio of two lengths" (whatever that means) always be named? Is there a number? For, numbers have names. 5Œ, 9.6, 
, 
. If we merely postulate that corresponding to every ratio, or every point on a line, there always is a number, then we will miss one of the most problematic questions in mathematics.
The interested reader is referred to The mathematical existence of numbers.
For more on the tension between a fraction and a ratio, see Section 3.
Please "turn" the page and do some Problems.
or
Continue on to Section 2: The relative sizes of fractions
