4.6: Multiplying and Dividing Mixed Fractions
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We begin with definitions of 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.
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.
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.=4⋅88+78 Equivalent fraction with LCD = 8.=4⋅8+78 Add numerators over common denominator.=398 Simplify the numerator.
Thus, 478 is equal to 39/8.
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.
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=4⋅8+78.
This is precisely what the third step in Example 1 looks like; we’re just eliminating a lot of the work.
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=4⋅3+23
Thus, the result is
423=143.
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.
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.
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.
Change 25/9 to a mixed fraction.
Solution
Break 25/9 into the appropriate sum.
259=189+79=2+79=279
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.
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.
Change 37/8 to a mixed fraction.
Solution
37 divided by 8 is 4, with a remainder of 5. That is:
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.
Change 38/9 to a mixed fraction.
- Answer
-
429
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.
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.
Simplify: −2112⋅245.
Solution
Change to improper fractions, factor, cancel, and simplify.
−2112⋅245=−2512⋅145 Change to improper fractions.=−25⋅1412⋅5 Multiply numerators; multiply denominators. Unlike signs; product is negative.=−(5⋅5)⋅(2⋅7)2⋅2⋅3)⋅(5) Prime factor.=−5⋅5⋅2⋅72⋅2⋅3⋅5 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,
−2112⋅245=−556.
Simplify:
−334⋅225
- Answer
-
−9
Simplify:
−445÷535.
Solution
Change to improper fractions, invert and multiply, factor, cancel, and simplify.
−445÷535=−245÷285 Change to improper fractions.=−245⋅528 Invert and multiply.=−2⋅2⋅2⋅35⋅52⋅2⋅7 Prime factor.=−2⋅2⋅2⋅33⋅52⋅2⋅7 Cancel common factors.=−67 Multiply numerators and denominators.⋅
Simplify:
−249⋅323
- 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. 117⋅212
26. 118⋅116
27. 4⋅116
28. 1710⋅4
29. (−1112)(334)
30. (−312)(313)
31. 712⋅1113
32. 214⋅1511
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