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4.6: Multiplying and Dividing Mixed Fractions

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We begin with definitions of proper and improper fractions.

Proper and Improper Fractions

A proper fraction is a fraction whose numerator is smaller than its denominator. An improper fraction is a fraction whose numerator is larger than its denominator.

For example,

23, 2339, and 119127

are all examples of proper fractions. On the other hand,

43, 317123, and 233101

are all examples of improper fractions.

A mixed fraction1 is part whole number, part fraction.

Mixed Fractions

The number

534

is called a mixed fraction. It is defined to mean

534=5+34.

In the mixed fraction 534, the 5 is the whole number part and the 3/4 is the fractional part.

Changing Mixed Fractions to Improper Fractions

We have all the tools required to change a mixed fraction into an improper fraction. We begin with an example.

Example 1

Change the mixed fraction 478 into an improper fraction.

Solution

We employ the definition of a mixed fraction, make an equivalent fraction for the whole number part, then add.

478=4+78  By definition.=488+78  Equivalent fraction with LCD = 8.=48+78  Add numerators over common denominator.=398  Simplify the numerator.

Thus, 478 is equal to 39/8.

Exercise

Change 534 to an improper fraction.

Answer

23/4

There is a quick technique you can use to change a mixed fraction into an improper fraction.

Quick Way to Change a Mixed Fraction to an Improper Fraction

To change a mixed fraction to an improper fraction, multiply the whole number part by the denominator, add the numerator, then place the result over the denominator.

Thus, to quickly change 478 to an improper fraction, multiply the whole number 4 by the denominator 8, add the numerator 7, then place the result over the denominator. In symbols, this would look like this:

478=48+78.

This is precisely what the third step in Example 1 looks like; we’re just eliminating a lot of the work.

Example 2

Change 423 to an improper fraction.

Solution

Take 423, multiply the whole number part by the denominator, add the numerator, then put the result over the denominator.

423=43+23

Thus, the result is

423=143.

Exercise

Change \(7 \frac{3{8}\) to an improper fraction.

Answer

59/8

It is very easy to do the intermediate step in Example 2 mentally, allowing you to skip the intermediate step and go directly from the mixed fraction to the improper fraction without writing down a single bit of work.

Example 3

Without writing down any work, use mental arithmetic to change 235 to an improper fraction.

Solution

To change 235 to an improper fraction, ignore the minus sign, proceed as before, then prefix the minus sign to the resulting improper fraction. So, multiply 5 times 2 and add 3. Put the result 13 over the denominator 5, then prefix the resulting improper fraction with a minus sign. That is,

235=135.

Exercise

Change 3512 to an improper fraction.

Answer

−41/12

Changing Improper Fractions to Mixed Fractions

The first step in changing the improper fraction 27/5 to a mixed fraction is to write the improper fraction as a sum.

275=255+25

Simplifying equation 4.1, we get

275=5+25=525

Comment. You can’t just choose any sum. The sum used in equation 4.1 is constructed so that the first fraction will equal a whole number and the second fraction is proper. Any other sum will fail to produce the correct mixed fraction. For example, the sum

275=235+45

is useless, because 23/5 is not a whole number. Likewise, the sum

275=205+75

is no good. Even though 20/5 = 4 is a whole number, the second fraction 7/5 is still improper.

Example 4

Change 25/9 to a mixed fraction.

Solution

Break 25/9 into the appropriate sum.

259=189+79=2+79=279

Exercise

Change 25/7 to a mixed fraction.

Answer

347.

Comment. A pattern is emerging. • In the case of 27/5, note that 27 divided by 5 is equal to 5 with a remainder of 2. Compare this with the mixed fraction result: 27/5=5 2 5 . • In the case of Example 4, note that 25 divided by 9 is 2 with a remainder of 7. Compare this with the mixed fraction result: 25/9=2 7 9 . These observations motivate the following technique.

Quick Way to Change an Improper Fraction to a Mixed Fraction

To change an improper fraction to a mixed fraction, divide the numerator by the denominator. The quotient will be the whole number part of the mixed fraction. If you place the remainder over the denominator, this will be the fractional part of the mixed fraction.

Example 5

Change 37/8 to a mixed fraction.

Solution

37 divided by 8 is 4, with a remainder of 5. That is:

Screen Shot 2019-09-03 at 9.24.33 PM.png

The quotient becomes the whole number part and we put the remainder over the divisor. Thus,

378=458.

Note: You can check your result with the “Quick Way to Change a Mixed Fraction to an Improper Fraction.” 8 times 4 plus 5 is 37. Put this over 8 to get 37/8.

Exercise

Change 38/9 to a mixed fraction.

Answer

429

Example 6

Change −43/5 to a mixed fraction.

Solution

Ignore the minus sign and proceed in the same manner as in Example 5. 43 divided by 5 is 8, with a remainder of 3.

Screen Shot 2019-09-03 at 9.27.31 PM.png

The quotient is the whole number part, then we put the remainder over the divisor. Finally, prefix the minus sign.

435=835

Multiplying and Dividing Mixed Fractions

You have all the tools needed to multiply and divide mixed fractions. First, change the mixed fractions to improper fractions, then multiply or divide as you did in previous sections.

1A mixed fraction is sometimes called a mixed number.

Example 7

Simplify: 2112245.

Solution

Change to improper fractions, factor, cancel, and simplify.

2112245=2512145  Change to improper fractions.=2514125  Multiply numerators; multiply denominators. Unlike signs; product is negative.=(55)(27)223)(5)  Prime factor.=55272235  Cancel common factors.=356  Multiply numerators and denominators.

This is a perfectly good answer, but if you want a mixed fraction answer, 35 divided by 6 is 5, with a remainder of 5. Hence,

2112245=556.

Exercise 4.6.1

Simplify:

334225

Answer

−9

Example 8

Simplify:

445÷535.

Solution

Change to improper fractions, invert and multiply, factor, cancel, and simplify.

445÷535=245÷285  Change to improper fractions.=245528  Invert and multiply.=222355227  Prime factor.=222335227  Cancel common factors.=67  Multiply numerators and denominators.

Exercise

Simplify:

249323

Answer

−2/3

Exercises

In Exercises 1-12, convert the mixed fraction to an improper fraction.

1. 213

2. 1811

3. 1119

4. 115

5. 137

6. 1317

7. 119

8. 1511

9. 112

10. 158

11. 113

12. 157


In Exercises 13-24, convert the improper fraction to a mixed fraction.

13. 137

14. 179

15. 135

16. 103

17. 165

18. 1613

19. 98

20. 165

21. 65

22. 1710

23. 32

24. 74


In Exercises 25-48, multiply the numbers and express your answer as a mixed fraction.

25. 117212

26. 118116

27. 4116

28. 17104

29. (1112)(334)

30. (312)(313)

31. 7121113

32. 2141511

33. (1213)(423)

34. (1114)(225)

35. (137)(334)

36. (145)(334)

37. 9(1215)

38. 4(256)

39. (218)(6)

40. (9)(316)

41. (412)(225)

42. (137)(334)

43. (216)4

44. (6)(119)

45. (1415)(212)

46. (115)(159)

47. (212)(1711)

48. (1711)(1712)


In Exercises 49-72, divide the mixed fractions and express your answer as a mixed fraction.

49. 8÷229

50. 423÷4

51. (312)÷(1116)

52. (125)÷(1115)

53. 612÷1712

54. 512÷1910

55. (4)÷(159)

56. (423)÷4

57. (523)÷(216)

58. (212)÷(229)

59. (612)÷(414)

60. (116)÷(118)

61. (6)÷(1311)

62. (623)÷(6)

63. (423)÷(4)

64. (623)÷(6)

65. (134)÷(1112)

66. (247)÷(115)

67. (523)÷119

68. 123÷129

69. (712)÷(225)

70. (513)÷(256)

71. (323)÷(119)

72. (812)÷(134)


73. Small Lots. How many quarter-acre lots can be made from 612 acres of land?

74. Big Field. A field was formed from 1712 half-acre lots. How many acres was the resulting field ?

75. Jewelry. To make some jewelry, a bar of silver 412 inches long was cut into pieces 112 inch long. How many pieces were made?

76. Muffins. This recipe will make 6 muffins: 1 cup milk, 123 cups flour, 2 eggs, 1/2 teaspoon salt, 112 teaspoons baking powder. Write the recipe for six dozen muffins.


Answers

1. 73

3. 2019

5. 107

7. 109

9. 32

11. 43

13. 167

15. 235

17. 315

19. 118

21. 115

23. 112

25. 267

27. 423

29. 4116

31. 8113

33. 5513

35. 5514

37. 1015

39. 1234

41. 1045

43. 823

45. 316

47. 4111

49. 335

51. 3517

53. 4219

55. 247

57. 2813

59. 1917

61. 457

63. 116

65. 1813

67. 5110

69. 318

71. 3310

73. 26 quarter-acre lots

75. 54 pieces


This page titled 4.6: Multiplying and Dividing Mixed Fractions is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by David Arnold.

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