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In real-world situations, quantities are accompanied by appropriate units so that the situations can be easily understood and interpreted. Before analyzing situations involving too large or too small quantities, their models are created by keeping the ratio between corresponding lengths constant. In this lesson, this ratio will be named and used to analyze real-life situations.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Explore
Scaling the Solar System
In the model of the Solar System below, the initial labeled distance corresponds to 93000000 miles. Zoom in or out to see how the labeled distance is affected.
Think about the following questions:
- Is it possible to make a model of the Solar System using the actual sizes of planets and distances between planets?
- Is it possible to draw a model of the Solar System on a sheet of paper? If so, how many miles should be represented by 1 inch?
Discussion
Scale
Example
Using the Scale to Find Actual Dimensions
The Sweden Solar System is the world's largest model of the planetary system. In this model, the Globe Arena in Stockholm represents the Sun, while the planets align to the north. Click on the dots below, representing the planets, to show the dimensions, location, and distance to the Globe Arena.
The scale used to make this model is 1:20 million.
a What is the actual diameter of the Earth? Give the answer in kilometers.
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b What is the actual distance from Pluto to the Sun?
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Solution
a Since the scale does not specify the units, it means that both numbers have the same units. Then, to find the actual diameter of the Earth, multiply both numbers by the diameter of the Earth in the model, 65 centimeters.
65 cm⋅1=20000000Original scale⋅65 cm⇕ 65cm=1300000000cm
This shows that 65 centimeters on the model corresponds to 1300000000 centimeters in real life. This means that the actual diameter of the Earth is 1300000000 centimeters. A more appropriate unit to represent this is kilometers, so next, use a conversion factor to convert the centimeters to kilometers.
1300000000cm
MultII
Multiply 1300000000cm by 100cm1m⋅1000m1km
1300000000cm⋅100cm1m⋅1000m1km
Simplify
CrossCommonFac
Cross out common factors
1300000000cm⋅100cm1m⋅1000m1km
CancelCommonFac
Cancel out common factors
1300000000⋅1001⋅10001km
MultFrac
Multiply fractions
1000001300000000km
CalcQuot
Calculate quotient
13000km
b Similarly, to find the actual distance from Pluto to the Sun, multiply both numbers in the scale by 300km.
300 km⋅1=20000000Original scale⋅300 km⇕300km=6000000000km
In conclusion, the distance from Pluto to the Sun is 6 billion kilometers.
Example
Calculating Population Density
The United States of America has an area of 9147420 square kilometers. In mid-2020, the total population was 331923317 people.
Answer
In the United States of America, the population density is 94 people per square mile. The population density measures the number of people per unit of area.
Hint
People per square mile can be written as mi2people. This implies that the population density is expressed as a ratio. Remember that 1 square kilometer is equal to 0.386102 square mile.
Solution
According to the problem, the population density needs to be given using the unit of people per square mile. This means that the population density is found by dividing the number of people by the area of a certain region.
The next step is to divide the population by the area, which gives the population density. Here, the result is rounded to the nearest integer.
The population density measures the number of people per unit of area.
Population: Area: 331923317people9147420km2
Before calculating the quotient, note that the area given is in square kilometers. Therefore, a unit conversion from square kilometers to square miles should be performed. To do this, use the fact that 1 square kilometer is equal to about 0.386102 square miles.
9147420km2
MultII
Multiply 9147420km2 by 1km20.386102mi2
9147420km2⋅1km20.386102mi2
Simplify
3531837.15684mi2
Pop. densityPop. density=3531837.15684mi2331923317people≈94people/mi2
Now that the population density is known, write the answer in a sentence form. The population density of the United States of America is approximately 94 people per square mile.
This statement assumes that the total population is spread out evenly across the United States.
Example
Practicing Converting a Scale
Mark wants to paint all of the walls of his bedroom except for the wall that contains the door. Each rectangular wall is 2.8 meters high. The paint he will use is sold in 5-liter cans — the price per liter is $25.
After painting for a few minutes, Mark noticed that 5 square meters can be covered with one liter of paint.
a What is the total area, in square meters, that Mark has to paint?
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b What is the minimum number of liters of paint Mark needs?
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c How many cans should Mark buy?
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d How many dollars could Mark save if the paint was sold in 1-liter cans?
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Hint
a Use the formula for the area of a rectangle to calculate the area of each wall.
b How many liters of paint are needed to cover one square meter of wall?
c Compare the number of liters needed to cover all three walls to the amount of paint in one can.
d Calculate the difference between the amount of money that Mark paid and the cost of 7 liters of paint.
Solution
a Start by writing the dimensions of the walls to be painted. Note that two walls have the same dimensions. This means that they also have the same area.
| Dimensions | |
|---|---|
| Wall 1 | ℓ1h=4.75m=2.8m
|
| Wall 2 | ℓ2h=4.75m=2.8m
|
| Wall 3 | ℓ3h=3m=2.8m
|
A3=(3m)(2.8m)=8.4m2
Now, to find the total area Mark has to paint, find the sum of the areas of all three walls.
Atotal=13.3+13.3+8.4=35m2
b Saying that 5 square meters can be covered with a liter of paint is equivalent to saying that 51 of a liter of paint can cover 1 square meter.
liter5m2⇔5m21liter
Multiplying 51liters/m2 by the total area gives the number of liters that Mark needs to paint the three walls.
35m2⋅5m21liter=7liters
c Mark needs 7 liters of paint, but each can of paint contains 5 liters. This means that Mark will actually have to buy 2 cans to have enough paint to cover all three walls.
d The cost of the 2 cans can be calculated by multiplying the number of liters in both cans by the price per liter. Each can contains 5 liters, so in total there are 10 liters of paint.
Price of 2 cans=10liters⋅liter25dollars=250dollars
If the paint was sold in 1-liter cans, Mark would need to buy 7 cans. In this case, he would pay 7⋅25=$175. The amount of money that Mark could have saved can be found by calculating the difference between the price of 10 liters of paint and 7 liters of paint.
250−175=$75
Pop Quiz
Practicing Finding or Using the Scale
On the applet, the model and the actual object are shown. Using the given information, find the scale or the size of either the model or the actual object.
- When entering the scale, write it in the form of a fraction with the numerator of 1.
- When entering the size of a model or an actual objects, round to the closest integer.
Closure
Calculating the Scale to Choose Better Phone Plan
Tadeo wants to buy a cheap phone plan for calling his friends and family. He asked Ali and Ramsha how much they pay per call.
- Ali said he paid $10 for his last call.
- Ramsha said she paid $8 for her last call.
Initially, Tadeo decided to take Ramsha's plan since she paid less. Later, he realized that this information is not helpful since he does not know the duration of each of the calls made by Ali and Ramsha.
Price per call is not an appropriate unit.
Therefore, Tadeo decided to ask Ali and Ramsha how long each phone call lasted.
- Ali said his call lasted 40 minutes.
- Ramsha said her call lasted 30 minutes.
Ali: Ramsha: $10→40minutes$8→30minutes
After this, Tadeo realized that dividing each call's cost by its duration will give him the price per minute, which is an appropriate unit to compare the plans. | Person | Scale of the Plan |
|---|---|
| Ali | 40min$10=$0.25 per minute |
| Ramsha | 30min$8≈$0.27 per minute |