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5 x = 20 16 size 12{ { {5} over {x} } = { {"20"} over {"16"} } } {} . Find the cross product.

5 16 = 20 x 80 = 20 x Divide the product 80 by the known factor 20. 80 20 = x 4 = x The unknown number is 4.

This means that 5 4 = 20 16 size 12{ { {5} over {4} } = { {"20"} over {"16"} } } {} , or, 5 is to 4 as 20 is to 6.

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16 3 = 64 x size 12{ { {"16"} over {3} } = { {"64"} over {x} } } {} Find the cross product.

16 x = 64 3 16 x = 192 Divide 192 by 16. x = 192 16 x = 12 The unknown number is 12.

The means that 16 3 = 64 12 size 12{ { {"16"} over {3} } = { {"64"} over {"12"} } } {} , or, 16 is to 3 as 64 is to 12.

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9 8 = x 40 size 12{ { {9} over {8} } = { {x} over {"40"} } } {} Find the cross products.

9 40 = 8 x 360 = 8 x Divide 360 by 8. 360 8 = x 45 = x The unknown number is 45.

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Practice set b

Find the unknown number in each proportion.

x 8 = 12 32 size 12{ { {x} over {8} } = { {"12"} over {"32"} } } {}

x = 3 size 12{x=3} {}

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7 x = 14 10 size 12{ { {7} over {x} } = { {"14"} over {"10"} } } {}

x = 5 size 12{x=5} {}

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9 11 = x 55 size 12{ { {9} over {"11"} } = { {x} over {"55"} } } {}

x = 45 size 12{x="45"} {}

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1 6 = 8 x size 12{ { {1} over {6} } = { {8} over {x} } } {}

x = 48 size 12{x="48"} {}

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Proportions involving rates

Recall that a rate is a comparison, by division, of unlike denominate numbers. We must be careful when setting up proportions that involve rates. The form is impor­tant. For example, if a rate involves two types of units, say unit type 1 and unit type 2, we can write

unit type 1 over unit type 2 equals unit type 1 over unit type 2. The same units appear on the same side, in this case, the same units are part of the same fraction.

or

unit type 1 over unit type 2 equals unit type 1 over unit type 2. The same units appear on the same side, in this case, the same unit is in both denominators and the same unit is in both numerators.

Both cross products produce a statement of the type

unit type 1 unit type 2 = unit type 1 unit type 2 size 12{ left ("unit type 1" right ) cdot left ("unit type 2" right )= left ("unit type 1" right ) cdot left ("unit type 2" right )} {}

which we take to mean the comparison

A comparison of types of units.

Examples of correctly expressed proportions are the following:

Two proportions. The first is miles over hr equals miles over hour, where the same unit is always in the denominator. The second is miles over miles equals hours over hours, where the same unit is in its own fraction.

However, if we write the same type of units on different sides, such as,

unit type 1 unit type 2 = unit type 2 unit type 1 size 12{ { {"unit type 1"} over {"unit type 2"} } = { {"unit type 2"} over {"unit type 1"} } } {}

the cross product produces a statement of the form

A chart showing the comparison of different unit types.

We can see that this is an incorrect comparison by observing the following example: It is incorrect to write

2 hooks 3 poles = 6 poles 4 hooks size 12{ { {"2 hooks"} over {"3 poles"} } = { {"6 poles"} over {"4 hooks"} } } {}

for two reason.

  1. The cross product is numerically wrong: 2 4 3 6 size 12{ left (2 cdot 4<>3 cdot 6 right )} {} .
  2. The cross product produces the statement “hooks are to hooks as poles are to poles,” which makes no sense.

Exercises

A statement that two ratios or are equal is called a .

rates, proportion

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For the following 9 problems, write each proportion in fractional form.

3 is to 7 as 18 is to 42.

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1 is to 11 as 3 is to 33.

1 11 = 3 33 size 12{ { {1} over {"11"} } = { {3} over {"33"} } } {}

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9 is to 14 as 27 is to 42.

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6 is to 90 as 3 is to 45.

6 90 = 3 45 size 12{ { {6} over {"90"} } = { {3} over {"45"} } } {}

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5 liters is to 1 bottle as 20 liters is to 4 bottles.

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18 grams of cobalt is to 10 grams of silver as 36 grams of cobalt is to 20 grams of silver.

18   gr cobalt 10   gr silver = 36   gr cobalt 20   gr silver size 12{ { {"18"" gr cobalt"} over {"10"" gr silver"} } = { {"36"" gr cobalt"} over {"20"" gr silver"} } } {}

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4 cups of water is to 1 cup of sugar as 32 cups of water is to 8 cups of sugar.

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3 people absent is to 31 people present as 15 peo­ple absent is to 155 people present.

3   people absent 31 people present = 15   people absent 155   people present size 12{ { {3" people absent"} over {"31 people present"} } = { {"15"" people absent"} over {"155"" people present"} } } {}

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6 dollars is to 1 hour as 90 dollars is to 15 hours.

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For the following 10 problems, write each proportion as a sentence.

3 4 = 15 20 size 12{ { {3} over {4} } = { {"15"} over {"20"} } } {}

3 is to 4 as 15 is to 20

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1 8 = 5 40 size 12{ { {1} over {8} } = { {5} over {"40"} } } {}

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3 joggers 100   feet = 6 joggers 200   feet size 12{ { {"3 joggers"} over {"100"" feet"} } = { {"6 joggers"} over {"200"" feet"} } } {}

3 joggers are to 100 feet as 6 joggers are to 200 feet

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12 marshmallows 3   sticks = 36 marshmallows 9   sticks size 12{ { {"12 marshmallows"} over {3" sticks"} } = { {"36 marshmallows"} over {9" sticks"} } } {}

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40 miles 80 miles = 2 gallons 4 gallons size 12{ { {"40 miles"} over {"80 miles"} } = { {"2 gallons"} over {"5 gallons"} } } {}

40 miles are to 80 miles as 2 gallons are to 4 gallons

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4 couches 10   couches = 2 houses 5 houses size 12{ { {"4 couches"} over {"10"" couches"} } = { {"2 houses"} over {"5 houses"} } } {}

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1 person 1 job = 8 people 8 jobs size 12{ { {"1 person"} over {"1 job"} } = { {"8 people"} over {"8 jobs"} } } {}

1 person is to 1 job as 8 people are to 8 jobs

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1 popsicle 2   children = 1 2 popsicle 1 child size 12{ { {"1 popsicle"} over {2" children"} } = { { { {1} over {2} } " popsicle"} over {"1 child"} } } {}

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2,000 pounds 1   ton = 60,000 pounds 30   tons size 12{ { {"2,000 pounds"} over {1" ton"} } = { {"60,000 pounds"} over {"30"" tons"} } } {}

2,000 pounds are to 1 ton as 60,000 pounds are to 30 tons

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1   table 5 tables = 2 people 10 people size 12{ { {1" table"} over {"5 table"} } = { {"2 people"} over {"10 people"} } } {}

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For the following 10 problems, solve each proportion.

x 5 = 6 15 size 12{ { {x} over {5} } = { {6} over {"15"} } } {}

x = 2 size 12{x=2} {}

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x 10 = 28 40 size 12{ { {x} over {"10"} } = { {"28"} over {"40"} } } {}

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5 x = 10 16 size 12{ { {5} over {x} } = { {"10"} over {"16"} } } {}

x = 8 size 12{x=8} {}

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13 x = 39 60 size 12{ { {"13"} over {x} } = { {"39"} over {"60"} } } {}

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1 3 = x 24 size 12{ { {1} over {3} } = { {x} over {"24"} } } {}

x = 8 size 12{x=8} {}

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7 12 = x 60 size 12{ { {7} over {"12"} } = { {x} over {"60"} } } {}

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8 3 = 72 x size 12{ { {8} over {3} } = { {"72"} over {x} } } {}

x = 27 size 12{x="27"} {}

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16 1 = 48 x size 12{ { {"16"} over {1} } = { {"48"} over {x} } } {}

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x 25 = 200 125 size 12{ { {x} over {"25"} } = { {"200"} over {"125"} } } {}

x = 40 size 12{x="40"} {}

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65 30 = x 60 size 12{ { {"65"} over {"30"} } = { {x} over {"60"} } } {}

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For the following 5 problems, express each sentence as a proportion then solve the proportion.

5 hats are to 4 coats as x size 12{x} {} hats are to 24 coats.

x = 30 size 12{x="30"} {}

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x size 12{x} {} cushions are to 2 sofas as 24 cushions are to 16 sofas.

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1 spacecraft is to 7 astronauts as 5 spacecraft are to x size 12{x} {} astronauts.

x = 35 size 12{x="35"} {}

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56 microchips are to x circuit boards as 168 microchips are to 3 circuit boards.

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18 calculators are to 90 calculators as x size 12{x} {} students are to 150 students.

x = 30 size 12{x="30"} {}

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x size 12{x} {} dollars are to $40,000 as 2 sacks are to 1 sack.

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Indicate whether the proportion is true or false.

3 16 = 12 64 size 12{ { {3} over {"16"} } = { {"12"} over {"64"} } } {}

true

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2 15 = 10 75 size 12{ { {2} over {"15"} } = { {"10"} over {"75"} } } {}

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1 9 = 3 30 size 12{ { {1} over {9} } = { {3} over {"30"} } } {}

false

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6 knives 7 forks = 12 knives 15   forks size 12{ { {"6 knives"} over {"7 forks"} } = { {"12 knives"} over {"15"" forks"} } } {}

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33 miles 1 gallon = 99 miles 3 gallons size 12{ { {"33 miles"} over {"1 gallon"} } = { {"99 miles"} over {"3 gallons"} } } {}

true

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320 feet 5 seconds = 65 feet 1 second size 12{ { {"320 feet"} over {"5 seconds"} } = { {"65 feet"} over {"1 second"} } } {}

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35 students 70 students = 1 class 2 classes size 12{ { {"35 students"} over {"70 students"} } = { {"1 class"} over {"2 classes"} } } {}

true

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9 ml chloride 45 ml chloride = 1 test tube 7   test tubes size 12{ { {"9 ml chloride"} over {"45"" ml chloride"} } = { {"1 test tube"} over {7" test tubes"} } } {}

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Exercises for review

( [link] ) Use the number 5 and 7 to illustrate the commutative property of addition.

5 + 7 = 12 7 + 5 = 12 alignl { stack { size 12{5+7="12"} {} #size 12{7+5="12"} {} } } {}

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( [link] ) Use the numbers 5 and 7 to illustrate the commutative property of multiplication.

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( [link] ) Find the difference. 5 14 3 22 size 12{ { {5} over {"14"} } - { {3} over {"22"} } } {} .

17 77 size 12{ { {"17"} over {"77"} } } {}

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( [link] ) Find the product. 8 . 06129 1, 000 size 12{8 "." "06129" cdot 1,"000"} {} .

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( [link] ) Write the simplified fractional form of the rate “sixteen sentences to two paragraphs.”

8   sentences 1   paragraph size 12{ { {8" sentences"} over {1" paragraph"} } } {}

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Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
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you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
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Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, Fundamentals of mathematics. OpenStax CNX. Aug 18, 2010 Download for free at http://cnx.org/content/col10615/1.4
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