11. Scale Drawings
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Chapter 9
11.
Scale Drawings
Scale drawings are used to represent objects that are too large or too small to be drawn to their actual size. They maintain proportions through a scale factor, showing the relationship between the real size and the drawn size. This lesson covers how to work with scale, including calculating the length scale factor and using it in drawings and models to create accurate representations.
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Lesson Settings & Tools
| | 12 Theory slides |
| | 12 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Scale Drawings
Slide of 12
It is sometimes utterly impossible to accurately model an object or real-life scenario using the original dimensions. Here, scale drawings and models come into action. This lesson will expand on how these tools are instrumental in modeling some common cases.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Challenge
Maps and Actual Distances
On a sunny Saturday morning, Kevin — a boy with a passion for formula 1 racing cars — wakes up and finds a letter sitting on the table. It is from his pops.
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Discussion
The Scale of a Map
Most maps include the math of a particular relation between two units of measure. One of the measures refers to distances on the map itself and the other refers to actual distances. This relationship is called a scale. Other real-life tools use this relationship as well.
Concept
Scale
The scale of a model or drawing is the ratio between any length on the model or drawing and its corresponding length on the actual object or place.
Length on the drawing:Corresponding length on the actual object
Suppose a drawing has a scale of 1in : 100ft. This means that 1 inch on the drawing represents 100 feet on the actual object. Apart from the colon notation, a scale can be expressed using an equals sign or as a fraction, as it is a ratio.
| Denoting a Scale | |
|---|---|
| Ratio | 1in:100ft |
| Equals Sign | 1in=100ft |
| Fraction | 100ft1in |
When a scale is written without specifying the units, it is understood that both numbers have the same units of measure. For example, a scale of 1:2 means that the actual object is twice the size of the model. A scale of 1:0.5 means that the actual object is half the size of the model.
External credits: Derek Quinn
Discussion
Scale Drawing
A scale drawing is a two-dimensional drawing that is similar to an actual object or place. In a scale drawing, the ratio of any length on the drawing to the actual length is always the same and equal to the scale of the drawing.
actual →drawing →L1ℓ1=L2ℓ2← actual ← drawing
Possible examples of scale drawing are floor plans, blueprints, and maps. 
90 yd1.5 in=ℓ15.65 in
By solving this equation for ℓ, the distance Kevin walked from his house to his uncle's bakery can be found.
90 yd1.5 in=ℓ15.65 in
ℓ=939 yd
Example
Finding a Missing Length
After correctly determining the distance traveled to the bakery, Kevin's uncle trades him the tape measure for a smartwatch and a new map. Kevin's next task is to reach his aunt's house. There is one catch — his route has to pass through the local bank.
Kevin makes it to his aunt's house! She asks him how is her house from the bank on the map in inches? Kevin has to answer correctly to continue the scavenger hunt.
Since a map is a scale drawing, the ratio of any length on the drawing to the actual length always remains the same. Based on that, the following equation is created.
On the map, the bank and the aunt's house are 2.66 inches apart. 
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Hint
A map is a scale drawing. Then, the ratio of any length on the drawing to the actual length is always the same. The watch shows the distances traveled, where the first distance indicates the distance between the bakery and the bank.
Solution
Kevin's smartwatch gives the actual distances between the visited places. According to the map, the bakery and the bank are 2.85 inches apart. Let x be the distance between the bank and the aunt's house on the map.
| Place 1 | Place 2 | Distance on the Map (in) | Actual Distance (mi) |
|---|---|---|---|
| Bakery | Bank | 2.85 | 0.75 |
| Bank | Aunt's house | x | 0.70 |
actual→drawing→ 0.752.85=0.70x ←actual ←drawing
The value of x can be found by solving this equation.
0.752.85=0.70x
▼
Solve for x
MultEqn
LHS⋅0.70=RHS⋅0.70
0.752.85⋅0.70=x
CalcQuot
Calculate quotient
3.8⋅0.70=x
Multiply
Multiply
2.66=x
RearrangeEqn
Rearrange equation
x=2.66
Discussion
Scale for Three Dimensional Objects
If the original real-life situation involves a three-dimensional object, making a scale model is more useful than a drawing. The idea behind a scale model is the same as a scale drawing, but the model has three dimensions instead of two.
Concept
Scale Model
A scale model is a three-dimensional model that is similar to a three-dimensional object. The ratio of a linear measurement of a model to the corresponding linear measurement of the actual object is always the same and equal to the scale of the model.
actual →model →L1ℓ1=L2ℓ2← actual← model
Here is an example scale model of a building. 
Example
Height of a Lighthouse
The lighthouse door is closed and the code to open it is the actual height of the lighthouse, measured in meters. For a moment, Kevin does not know what to do. He realizes that he should open the backpack his aunt gave him. Inside, he finds a scale model of the lighthouse and a tape measure.
External credits: Ekayasa.Design
Kevin figures it out and manages to measure the width of the base of the lighthouse. Also, he measures the width and height of the scale model with the tape measure.
| Scale Model | Lighthouse | |
|---|---|---|
| Width | 10 cm | 5.25 m |
| Height | 30 cm | ? |
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Hint
In a scale model, the ratio of a linear measurement on the model to the corresponding linear measurement on the actual object is always the same.
Solution
Let x be the height of the lighthouse. In a scale model, the ratio of a linear measurement on the model to the corresponding linear measurement on the actual object is always the same. This leads to writing the following equation.
The height of the lighthouse is 15.75 meters. Kevin is ready to enter the lighthouse.
actual →model →5.25 m10 cm=x30 cm← actual← model
The height of the lighthouse can be found by solving the previous equation for x.
5.2510=x30
CrossMult
Cross multiply
10x=5.25⋅30
Multiply
Multiply
10x=157.5
DivEqn
LHS/10=RHS/10
x=10157.5
CalcQuot
Calculate quotient
x=15.75
Discussion
Length Scale Factor
The length scale factor of a scale drawing or scale model is the ratio of a length on the drawing or model to the corresponding actual length where both lengths have the same units of measure.
Length scale factor=Actual lengthLength on model
Example
Finding Length Scale Factors
At the top of the lighthouse, Kevin finds a blueprint of his room and a scale model of his house shed. He understands that he must return home.
In his room, Kevin finds a chest closed with a padlock. The pin to open it is 156 times the sum of the length scale factors of the blueprint and the scale shed. 
Kevin uses the tape measure and finds that his bed is 1.80 meters long. He then goes to the backyard and measures the width of the shed. It is 4.50 meters wide. What is the pin of the padlock?
Before finding the length scale factor, all the dimensions should be written with the same units of measure. Use the fact that 1 meter is the same as 100 centimeters to convert 1.80 meters to centimeters.
As before, write all the dimensions using the same units of measure.
The pin to open the padlock is 13.
External credits: juicy_fish
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Hint
Be sure all the dimensions are written with the same units of measure before finding the length scale factor.
Solution
The length scale factor of the blueprint is the ratio of a dimension on the blueprint to the corresponding actual dimension. The dimensions of the bed on the blueprint are written on its side. Additionally, Kevin measured his actual bed length.
| Blueprint Dimensions (cm) | Actual Dimensions (m) | |
|---|---|---|
| Bed length | 9 | 1.80 |
| Bed width | 5 |
1.80 m⋅1 m100 cm=180 cm
Next, divide the bed length on the blueprint by the actual length of the bed to find the scale factor.
The length scale factor of the blueprint is 201. This means that the actual dimensions of the bed — and the entire room — were divided by 20 to make the blueprint. Similarly, the length scale factor of the scale shed can be found. The width and length of the model are known and Kevin measured the actual width of the shed.
| Scale Model Dimensions (cm) | Actual Dimensions (m) | |
|---|---|---|
| Length | 20 | |
| Width | 15 | 4.50 |
4.50 m⋅1 m100 cm=450 cm
The length scale factor is the quotient between the width of the scale shed and the actual width of the shed.
Now that both length scale factors are known, the pin of the padlock can be found. Recall, it is 156 times the sum of the length scale factors.
Pin=156(scale factor 1+scale factor 2)
SubstituteValues
Substitute values
Pin=156(201+301)
▼
Simplify
ExpandFrac
ba=b⋅30a⋅30
Pin=156(20⋅301⋅30+301)
ExpandFrac
ba=b⋅20a⋅20
Pin=156(20⋅301⋅30+30⋅201⋅20)
Multiply
Multiply
Pin=156(60030+60020)
AddFrac
Add fractions
Pin=156⋅60050
ReduceFrac
ba=b/50a/50
Pin=156⋅121
MoveLeftFacToNumOne
a⋅b1=ba
Pin=12156
CalcQuot
Calculate quotient
Pin=13
Example
Next Stop
Kevin eagerly opens the chest. He discovers a tablet! Upon powering it on, a map with a marker at his current position and another marker at his next stop appear. Kevin then presses the navigate button. Oh no! The app locks and asks him what the actual distance is between the marked places, in yards.
According to the information given by the app, with the current zoom, the markers are 9 inches apart on the screen and the length scale factor is 0.0004. What pin unlocks the app?
External credits: Freepik
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Hint
The ratio of the distance on the tablet to the actual distance is equal to the length scale factor. Use the fact that 36 inches are the same as 1 yard.
Solution
Since a map is a scale drawing, the ratio of any length on the map to the actual length is always the same and equal to the length scale factor.
The places are 22500 inches apart. This can be converted to yards by using the fact that 36 inches are the same as 1 yard.
Corresponding actual lengthLength in map=k
This means that the ratio of the distance between the marked places on the tablet to the actual distance is equal to the length scale factor. Let x be the actual distance. According to the information given by the app, the places are 9 inches apart on the screen and the scale factor is 0.0004.
Actual lengthLength in map=k
SubstituteValues
Substitute values
x9 in=0.0004
▼
Solve for x
MultEqn
LHS⋅x=RHS⋅x
9 in=0.0004⋅x
DivEqn
LHS/0.0004=RHS/0.0004
0.00049 in=x
CalcQuot
Calculate quotient
22500 in=x
RearrangeEqn
Rearrange equation
x=22500 in
22500 in⋅36 in1 yd=625 yd
The actual distance between the marked places is 625 yards. This means that the pin that unlocks the app is 625. Enter it to see the route and follow Kevin's adventure.
Example
Length in a Scale Model
When Kevin arrived at the last destination, his father was there waiting with a huge smile and a box in his hands. Inside, there was a scale formula 1 racing car made with a length scale factor of 161!
External credits: kjpargeter
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Solution
Let x be the length of the scale car, in centimeters. Since the length of the actual car is given in meters, convert it to centimeters first.
The length of the scale formula 1 is 35 centimeters.
5.6 m⋅1 m100 cm=560 cm
The actual formula 1 car is 560 centimeters long. The ratio of the length of the scale car to the length of the actual car is equal to the length scale factor, 161.
Length Scale factor=560x
Substitute
Length Scale factor=161
161=560x
▼
Solve for x
MultEqn
LHS⋅560=RHS⋅560
161⋅560=x
MoveRightFacToNumOne
b1⋅a=ba
16560=x
CalcQuot
Calculate quotient
35=x
RearrangeEqn
Rearrange equation
x=35
Closure
Relating the Area of a Scale Drawing to the Actual Area
The length scale factor gives the relationship between the dimensions of a scale drawing and the original drawing. Kevin wonders whether the areas are also related somehow. Use the following applet to investigate it.
In the following table, some ratios of the area of the scale triangle to the area of the original triangle are computed.
Notice that the ratio of the scale triangle's area to the original area is equal to the square of the length scale factor. This relationship holds for any scale drawing. Finally, think about whether there is also a relationship between a scale model's volume and the actual object's volume.
| Area of Original Triangle | Length Scale Factor | Area of Scale Triangle | Ratio of Areas |
|---|---|---|---|
| 2.8 | 2 | 11.2 | 2.811.2=4=22 |
| 2.8 | 0.5=21 | 0.7 | 2.80.7=0.25=41=(21)2 |
| 2.8 | 1.5=23 | 6.3 | 2.86.3=2.25=49=(23)2 |
Scale Drawings
Exercise 1.1
Find the actual distance between the mentioned places.
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We begin by noticing that a map is a scale drawing and so, the ratio of any distance on the map to the actual distance is always the same and equal to the scale of the map. First, let's write the scale of the map. Scale of the Map 4in : 37mi The scale can also be written as a ratio as 437. We are told that Reno and Sacramento are 12 inches apart on the map. Let x be the actual distance between the cities, in miles. Next, we write the following proportion. map→/actual→ 12/x = 4/37 ←map/←actual Let's solve the equation for x.
We have that Reno and Sacramento are 111 miles apart.
As in Part A, we will use the fact that a map is a scale drawing and then the ratio of any distance on the map to the actual distance is always the same and equal to the scale of the map. Let's begin by writing the scale of the map.
Scale of the Map 1.4inches = 19miles
Let's write the scale as a ratio as 1.419. We are told that San Antonio and Houston are 14 inches apart on the map. Let x be the actual distance between the cities, in miles. We can write the following proportion.
map→/actual→ 14/x = 1.4/19 ←map/←actual
Let's solve the equation for x.
Our calculations show that San Antonio and Houston are 190 miles apart.
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Consider the following scale drawing and model.
What is the actual height of the fire extinguisher?
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What is the height of the actual pyramid?
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We are given the scale drawing of a fire extinguisher which is set to a scale of 3in:1ft. We can write this scale as a ratio. Scale of the drawing 3/1 The ratio of the height of the drawing to the actual height is equal to the scale of the drawing. The height of the fire extinguisher on the drawing is 5.25 inches. Let x be the height of the actual fire extinguisher in feet. Using this information we can write the following proportion. 5.25/x = 3/1 Let's solve the equation for x.
The height of the actual fire extinguisher is 1.75 feet.
We are given the scale model of a pyramid which is set to a scale of 2cm = 15m. Let's begin by writing this scale as a ratio.
Scale of the model 2/15
The ratio of the height of the model to the actual height is equal to the scale of the model. The height of the pyramid on the model is 8 centimeters. Let x be the height of the actual pyramid. We can write a proportion relating x to the scale of the model.
8/x = 2/15
Let's solve the equation for x to find the height of the actual pyramid.
The height of the actual pyramid is 60 meters.
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Find the scale of each map.
The building and the factory are 32 centimeters apart on the map. They are 6 kilometers apart in real life.
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On a map, Los Angeles and New York are 20 inches apart. In real life, these cities are 2476 miles apart.
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We begin by recalling that the scale of a scale drawing is the ratio between any length on the drawing to its corresponding actual length. In our case, we are given both distances: that of the building and the factory on the map and the actual distance in real life.
| Distance Between the Building and the Factory | |
|---|---|
| Distance on Map | Actual Distance |
| 32cm | 6km |
We have all what we need to find the scale of the map. Let's find the ratio of the distance on the map to the actual distance.
The scale of the map is 16cm3km or 16cm:3km. This means that 16 centimeters on the map represent 3 kilometers.
As in the previous part, we can find the scale of the map by dividing a distance on the map by the corresponding actual distance.
Scale = Distance on Map/Actual Distance
We are told that Los Angeles and New York are 20 inches apart on the map and 2476 miles apart in real life. Let's use this data to find the scale of the map.
The scale of the map can also be written as 5in:619mi. This means that 5 inches on the map are the same as 619 miles in real life.
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Select the appropriate answer.
A landscape designer is going through the process of creating a garden. She made a scale model of the fence she plans to use.
External credits: @macrovector-official
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A soccer goalpost is 24 feet long and 8 feet high.
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Let's find the scale of the model by dividing the length of the fence on the model by the length of the actual fence.
The scale used for making the fence is 2in : 13ft. 1in & : 13ft && * 2in & : 13ft && ✓ 3in & : 13ft && * 13in & : 2ft && *
First, let's find the scale used for making the soccer goalpost. We can find it by dividing the height of the model by the height of an actual soccer goalpost. Likewise, we can find it by dividing the length of the model by the length of the actual goalpost.
The scale used for making the soccer goalpost is 3in:4ft. As we can see, this is one of the given scales. 3in & : 4ft ✓ 6in & : 8ft 9in & : 12ft 18in & : 8ft If we multiply both sides of the scale by 2 and by 3, we will get the second and third given scales. 2* 3in & : 2* 4ft ⇒& 6in & : 8ft && ✓ 3* 3in & : 3* 4ft ⇒& 9in & : 12ft && ✓ However, there is no way to rewrite the scale we obtained as the given fourth scale. This means that the fourth scale was not used when making the model. 3in & : 4ft && ✓ 6in & : 8ft && ✓ 9in & : 12ft && ✓ 18in & : 8ft && * Also, according to the last scale, 18 inches represents 8 feet, implying that the soccer goalpost is 8 feet long, which is wrong.
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A scale drawing of a tennis court is shown.
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Let's begin by writing the dimensions of the tennis court on the scale drawing. Length &= 13in Width &= 6in The scale used is 1in:6ft which means that 1 inch on the drawing represents 6 feet in real life. Therefore, 6 inches on the drawing represents 6* 6 = 36 feet in real life, and 13 inches on the drawing represents 13* 6 = 78 feet in real life. We are ready to write the dimensions of the actual tennis court. 78ft* 36ft
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A map of New York has a scale of 3 cm:7 mi. The linear distance from the Newark Liberty International Airport to the John F. Kennedy International Airport is 21 miles. What is the distance between the airports on the map?
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Let d be the distance between the airports on the map. We are told that the actual distance between the airports is 21 miles. Also, we know the scale of the map is 3cm:7in. Let's write the scale as a fraction. scale = 3cm/7mi Since a map is a scale drawing, the ratio of the distance d between the airports on the map to the actual distance 21 miles is equal to the scale of the map. d/21mi = 3cm/7mi Let's solve the equation for d to find the distance between the airports on the map.
On the map, the airports are 9 centimeters apart.
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The distance between places A and B in real life is 105 yards.
External credits: Macrovector
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Let x be the distance between C and D on the map. Let's make a table summarizing the given information.
| Place 1 | Place 2 | Distance on Map (cm) | Actual Distance (yd) |
|---|---|---|---|
| A | B | 1.4 | 105 |
| C | D | x | 180 |
Since a map is a scale drawing, the ratio of any length on the drawing to the actual length is always the same. drawing→/actual→ l_1/L_1 = l_2/L_2 ←drawing/←actual The ratio of the distance between A and B on the map to the actual distance between A and B is equal to the ratio of the distance between C and D on the map to their actual distance. 1.4/105 = x/180 Let's solve the equation for x.
Places C and D are 2.4 centimeters apart on the map.
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