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2.6 Ratio and Proportion Applications

Learning Objectives

  1. Understand the difference between a ratio and a proportion.
  2. Solve proportions using cross multiplication.
  3. Solve applications involving proportions, including similar triangles.

Definitions

A ratioRelationship between two numbers or quantities usually expressed as a quotient. is a relationship between two numbers or quantities usually expressed as a quotient. Ratios are typically expressed using the following notation:

All of the above are equivalent forms used to express a ratio. However, the most familiar way to express a ratio is in the form of a fraction. When writing ratios, it is important to pay attention to the units. If the units are the same, then the ratio can be written without them.

 

Example 1: Express the ratio 12 feet to 48 feet in reduced form.

Solution:

Answer: 1 to 4

 

If the units are different, then we must be sure to include them because the ratio represents a rateA ratio where the units for the numerator and the denominator are different..

 

Example 2: Express the ratio 220 miles to 4 hours in reduced form.

Solution:

Answer: 55 miles to 1 hour (or 55 miles per hour)

 

Rates are useful when determining unit costThe price of each unit., or the price of each unit. We use the unit cost to compare values when the quantities are not the same. To determine the unit cost, divide the cost by the number of units.

 

Example 3: A local supermarket offers a pack of 12 sodas for $3.48 on sale, and the local discount warehouse offers the soda in a 36-can case for $11.52. Which is the better value?

Solution: Divide the cost by the number of cans to obtain the unit price.

Supermarket Discount warehouse
$3.4812cans=$0.29/can $11.5236cans=$0.32/can

Answer: The supermarket sale price of $3.48 for a 12-pack is a better value at $0.29 per can.

 

A proportionA statement of the equality of two ratios. is a statement of equality of two ratios.

This proportion is often read “a is to b as c is to d.” Here is an example of a simple proportion,

If we clear the fractions by multiplying both sides of the proportion by the product of the denominators, 8, then we obtain the following true statement:

Given any nonzero real numbers a, b, c, and d that satisfy a proportion, multiply both sides by the product of the denominators to obtain the following:

This shows that cross products are equal, and is commonly referred to as cross multiplicationIf ab=cd, then ad=bc..

Solving Proportions

Cross multiply to solve proportions where terms are unknown.

 

Example 4: Solve: 58=n4.

Solution: Cross multiply and then solve for n.

Answer: n=52

 

Example 5: Solve: 15x=56.

Solution: Cross multiply then solve for x.

Answer: x=18

 

Example 6: Solve: n+35=72.

Solution: When cross multiplying, be sure to group n+3. Apply the distributive property in the next step.

Answer: n=292

 

Try this! Solve: 53=3n12.

Answer: n=139

Video Solution

(click to see video)

Applications

When setting up proportions, consistency with the units of each ratio is critical. Units for the numerators should be the same and units for the denominators should also be the same.

 

Example 7: It is claimed that 2 out of 3 dentists prefer a certain brand of toothpaste. If 600 dentists are surveyed, then how many will say they prefer that brand?

Solution: First, identify the unknown and assign it a variable.

Since you are looking for the number of dentists who prefer the brand name out of a total of 600 surveyed, construct the ratios with the number of dentists who prefer the brand in the numerator and the total number surveyed in the denominator.

Cross multiply and solve for n,

Answer: The claim suggests that 400 out of 600 dentists surveyed prefer the brand name.

 

Example 8: In Tulare County, 3 out of every 7 voters said yes to Proposition 40. If 42,000 people voted, how many said no to Proposition 40?

Solution: The problem gives the ratio of voters who said yes, but it asks for the number who said no.

If 3 out of 7 said yes, then we can assume 4 out of 7 said no. Set up the ratios with the number of voters who said no in the numerator and the total number of voters in the denominator.

Cross multiply and solve for n.

Answer: 24,000 voters out of 42,000 said no.

 

Example 9: The sum of two integers in the ratio of 4 to 5 is 27. Find the integers.

Solution: The sum of two integers is 27; use this relationship to avoid two variables.

The integers are given to be in the ratio of 4 to 5. Set up the following proportion:

Use 27 − n to determine the other integer.

Answer: The integers are 12 and 15.

 

Try this! A recipe calls for 5 tablespoons of sugar for every 8 cups of flour. How many tablespoons of sugar are required for 32 cups of flour?

Answer: 20 tablespoons of sugar

Video Solution

(click to see video)

Similar Triangles

We will often encounter proportion problems in geometry and trigonometry. One application involves similar trianglesTriangles with the same shape but not necessarily the same size. The measures of corresponding angles are equal and the corresponding sides are proportional., which have the same shape, but not necessarily the same size. The measures of the corresponding angles of similar triangles are equal, and the measures of the corresponding sides are proportional. Given similar triangles ABC and RST,

We may write ABC ~ RST and conclude that all of the corresponding angles are equal. The notation indicates that angle A corresponds to angle R and that the measures of these angles are equal: A = R.

In addition, the measures of other pairs of corresponding angles are equal: B = S and C = T.

Use uppercase letters for angles and a lowercase letter to denote the side opposite of the given angle. Denote the proportionality of the sides as follows:

 

Example 10: If triangle ABC is similar to RST, where a=3, b=4, c=5, and r=9, then find the remaining two sides.

Solution: Draw a picture and identify the variables pictorially. Represent the remaining unknown sides by s and t. Set up proportions and solve for the missing sides.

Answer: The two remaining sides measure 12 units and 15 units.

 

The reduced ratio of any two corresponding sides of similar triangles is called the scale factorThe reduced ratio of any two corresponding sides of similar triangles.. In the previous example, the ratio of the two given sides a and r is

Therefore, triangle ABC is similar to triangle RST with a scale factor of 1/3. This means that each leg of triangle ABC is 1/3 of the measure of the corresponding legs of triangle RST. Also, another interesting fact is that the perimeters of similar triangles are in the same proportion as their sides and share the same scale factor.

 

Example 11: If a triangle ABC has a perimeter of 12 units and is similar to RST with a scale factor of 1/3, then find the perimeter of triangle RST.

Solution:

Scale factor 1/3 implies that the perimeters are in proportion to this ratio. Set up a proportion as follows:

Cross multiply and solve for x.

Answer: The perimeter of triangle RST is 36 units.

Key Takeaways

  • Solve proportions by multiplying both sides of the equation by the product of the denominators, or cross multiply.
  • When setting up a proportion, it is important to ensure consistent units in the numerators and denominators.
  • The corresponding angles of similar triangles are equal and their corresponding sides are proportional. The ratio of any two corresponding sides determines the scale factor, which can be used to solve many applications involving similar triangles.

Topic Exercises

Part A: Ratios and Rates

Express each ratio in reduced form.

1. 100 inches : 250 inches

2. 480 pixels : 320 pixels

3. 96 feet : 72 feet

4. 240miles4hours

5. 96feet3seconds

6. 6,000revolutions4 minutes

7. Google’s average 2008 stock price and earnings per share were $465.66 and $14.89, respectively. What was Google’s average price-to-earnings ratio in 2008? (Source: Wolfram Alpha)

8. The F-22 Raptor has two engines that each produce 35,000 pounds of thrust. If the takeoff weight of this fighter jet is 50,000 pounds, calculate the plane’s thrust-to-weight ratio. (Source: USAF)

9. A discount warehouse offers a box of 55 individual instant oatmeal servings for $11.10. The supermarket offers smaller boxes of the same product containing 12 individual servings for $3.60. Which store offers the better value?

10. Joe and Mary wish to take a road trip together and need to decide whose car they will take. Joe calculated that his car is able to travel 210 miles on 12 gallons of gasoline. Mary calculates that her car travels 300 miles on 19 gallons. Which of their cars gets more miles to the gallon?

Part B: Solving Proportions

Solve.

11. 23=n150

12. 7n=215

13. 13=5n

14. 125=6n

15. n8=32

16. n3=57

17. 8=2n3

18. 5n=30

19. 1=1n1

20. 1=1n+1

21. 40n=53

22. 2n+13=35

23. 53n+3=23

24. n+12n1=13

25. 5n+75=n12

26. 2n+3=n+76

27. Find two numbers in the ratio of 3 to 5 whose sum is 160. (Hint: Use n and 160 − n to represent the two numbers.)

28. Find two numbers in the ratio of 2 to 7 whose sum is 90.

29. Find two numbers in the ratio of −3 to 7 whose sum is 80.

30. Find two numbers in the ratio of −1 to 3 whose sum is 90.

31. A larger integer is 5 more than a smaller integer. If the two integers have a ratio of 6 to 5 find the integers.

32. A larger integer is 7 less than twice a smaller integer. If the two integers have a ratio of 2 to 3 find the integers.

Given the following proportions, determine each ratio, x:y.

33. x3=y4

34. x2y3=3y5

35. 2x+4y2x4y=32

36. x+yxy=35

Part C: Applications

Set up a proportion and then solve.

37. If 4 out of every 5 voters support the governor, then how many of the 1,200 people surveyed support the governor?

38. If 1 out of every 3 voters surveyed said they voted yes on Proposition 23, then how many of the 600 people surveyed voted yes?

39. Out of 460 students surveyed, the ratio to support the student union remodel project was 3 to 5. How many students were in favor of the remodel?

40. An estimated 5 out of 7 students carry credit card debt. Estimate the number of students that carry credit card debt out of a total of 14,000 students.

41. If the ratio of female to male students at the college is 6 to 5, then determine the number of male students out of 11,000 total students.

42. In the year 2009 it was estimated that there would be 838 deaths in the United States for every 100,000 people. If the total US population was estimated to be 307,212,123 people, then how many deaths in the United States were expected in 2009? (Source: CIA World Factbook)

43. In the year 2009 it was estimated that there would be 1,382 births in the United States for every 100,000 people. If the total US population was estimated to be 307,212,123 people, then how many births in the United States were expected in 2009? (Source: CIA World Factbook)

44. If 2 out of every 7 voters approve of a sales tax increase then determine the number of voters out of the 588 surveyed who do not support the increase.

45. A recipe calls for 1 cup of lemon juice to make 4 cups of lemonade. How much lemon juice is needed to make 2 gallons of lemonade?

46. The classic “Shirley Temple” cocktail requires 1 part cherry syrup to 4 parts lemon-lime soda. How much cherry syrup is needed to mix the cocktail given a 12-ounce can of lemon-lime soda?

47. A printer prints 30 pages in 1 minute. How long will it take to print a 720-page booklet?

48. A typist can type 75 words per minute. How long will it take to type 72 pages if there are approximately 300 words per page?

49. On a particular map, every 116 inch represents 1 mile. How many miles does 312 inches represent?

50. On a graph every 1 centimeter represents 100 feet. What measurement on the map represents one mile?

51. A candy store offers mixed candy at $3.75 for every half-pound. How much will 2.6 pounds of candy cost?

52. Mixed nuts are priced at $6.45 per pound. How many pounds of mixed nuts can be purchased with $20.00?

53. Corn at the farmers market is bundled and priced at $1.33 for 6 ears. How many ears can be purchased with $15.00?

54. If 4 pizzas cost $21.00, then how much will 16 pizzas cost?

55. A sweetened breakfast cereal contains 110 calories in one 34-cup serving. How many calories are in a 178-cup serving?

56. Chicken-flavored rice contains 300 calories in each 2.5-ounce serving. How many calories are in a 4-ounce scoop of chicken-flavored rice?

57. A 200-pound man would weigh about 33.2 pounds on the moon. How much will a 150-pound man weigh on the moon?

58. A 200-pound man would weigh about 75.4 pounds on Mars. How much will a 150-pound man weigh on Mars?

59. There is a 1 out of 6 chance of rolling a 1 on a six-sided die. How many times can we expect a 1 to come up in 360 rolls of the die?

60. There is a 1 out of 6 chance of rolling a 7 with two six-sided dice. How many times can we expect a 7 to come up in 300 rolls?

61. The ratio of peanuts to all nuts in a certain brand of packaged mixed nuts is 3 to 5. If the package contains 475 nuts, then how many peanuts can we expect?

62. A mixed bag of marbles is packaged with a ratio of 6 orange marbles for every 5 red marbles. If the package contains 216 orange marbles, then how many red marbles can we expect?

63. A graphic designer wishes to create a 720-pixel-wide screen capture. If the width to height ratio is to be 3:2, then to how many pixels should he set the height?

64. If a video monitor is produced in the width to height ratio of 16:9 and the width of the monitor is 40 inches, then what is the height?

Part D: Similar Triangles

If triangle ABC is similar to triangle RST, find the remaining two sides given the information.

65. a=6, b=8, c=10, and s=16

66. b=36, c=48, r=20, and t=32

67. b=2, c=4, r=6, and s=4

68. b=3, c=2, r=10, and t=12

69. a=40, c=50, s=3, and t=10

70. c=2, r=7, s=9, and t=4

71. At the same time of day, a tree casts a 12-foot shadow while a 6-foot man casts a 3-foot shadow. Estimate the height of the tree.

72. At the same time of day, a father and son, standing side by side, cast a 4-foot and 2-foot shadow, respectively. If the father is 6 feet tall, then how tall is his son?

73. If the 6-8-10 right triangle ABC is similar to RST with a scale factor of 2/3, then find the perimeter of triangle RST.

74. If the 3-4-5 right triangle ABC is similar to RST with a scale factor of 5, then find the perimeter of triangle RST.

75. An equilateral triangle with sides measuring 6 units is similar to another with scale factor 3:1. Find the length of each side of the unknown triangle.

76. The perimeter of an equilateral triangle ABC measures 45 units. If triangle ABC ~ RST and r=20, then what is the scale factor?

77. The perimeter of an isosceles triangle ABC, where the two equal sides each measure twice that of the base, is 60 units. If the base of a similar triangle measures 6 units, then find its perimeter.

78. The perimeter of an isosceles triangle ABC measures 11 units and its two equal sides measure 4 units. If triangle ABC is similar to triangle RST and triangle RST has a perimeter of 22 units, then find all the sides of triangle RST.

79. A 6-8-10 right triangle ABC is similar to a triangle RST with perimeter 72 units. Find the length of each leg of triangle RST.

80. The perimeter of triangle ABC is 60 units and b=20 units. If ABC ~ RST and s=10 units, then find the perimeter of triangle RST.

Part E: Discussion Board Topics

81. What is the golden ratio and where does it appear?

82. Research and discuss the properties of similar triangles.

83. Discuss the mathematics of perspective.

84. Research and discuss the various aspect ratios that are available in modern media devices.

Answers

1: 2:5

3: 4:3

5: 32 feet per second

7: 31.27

9: The discount warehouse

11: n=100

13: n=15

15: n=12

17: n=12

19: n=2

21: n=24

23: n=32

25: n=195

27: 60, 100

29: −60, 140

31: 25, 30

33: 3/4

35: 10

37: 960 people

39: 276 students

41: 5,000 male students

43: 4,245,672 births

45: 8 cups of lemon juice

47: 24 minutes

49: 56 miles

51: $19.50

53: 66 ears

55: 275 calories

57: 24.9 pounds

59: 60 times

61: 285 peanuts

63: 480 pixels

65: t = 20, r = 12

67: a = 3, t = 8

69: r = 8, b = 15

71: 24 feet

73: 36 units

75: 2 units

77: 30 units

79: r = 18 units, s = 24 units, t = 30 units