<meta http-equiv="refresh" content="1; url=/nojavascript/"> Proportions to Find Dimensions ( Read ) | Arithmetic | CK-12 Foundation
Dismiss
Skip Navigation

Proportions to Find Dimensions

%
Progress
Practice Proportions to Find Dimensions
Practice
Progress
%
Practice Now
Proportions to Find Dimensions

Have you ever thought about how displays are created in stores or supermarkets?

Jessica is in charge of creating a new project display at the supermarket. Her manager noticed her doodling in a notebook and was impressed with her artistic ability. He called Jessica into his office and handed her a design on a sheet of paper. “We want to create a special poster to focus on a key product each week. This is the sketch of the design. That product will be on sale and hopefully this new display will help people notice it and buy it,” Jessica’s manager explains. The design on the page is 8” \times 5”. It is a rectangle and the scale at the bottom of the page says 1” = 6”.

Jessica takes out some foamboard and a knife. She knows that she needs the scale to figure out the exact measurements of the display. The problem is that she can’t remember how to do the math. “If it were 1” to 1 ft,” she thinks, “Then the poster would be 8 ft \times 5 ft, but that isn’t it. 6" is only 1/2 of 1 ft, so this scale is 1" to 1/2 ft.” Jessica is puzzled. What are the dimensions for the display?

This Concept works on scale and proportions. You can use a scale to figure out the change in measurements from a picture to the actual thing.

Be ready so that you can help Jessica at the end of the Concept.

Guidance

Previously we worked on how you can find proportions in everyday life. This Concept focuses on scale and scale drawings. This is a key place where we use proportions in everyday life.

What is a scale drawing?

A scale drawing is a drawing that is used to represent an object that is too large to be drawn in its actual dimensions.

If you had a very tall building, you couldn’t make a drawing of the building that is as large as the building itself. Think about how many sheets of paper you would need to draw a 25 foot tall building. This is an example where a scale drawing would be very useful. We can use a scale to represent measurements and then draw the building in a size that makes more sense.

What is a scale? When we talk about scale, we aren’t talking about the object that we use to weigh things.

The scale that we are talking about is a fraction that shows the relationship between the measurement in a drawing and the measurement of a real object.

\frac{1\ inch}{4\ feet}

This scale says that we would use one inch to represent every four feet. The top number is the scale that we would use in the drawing. The bottom number represents the measurement of the actual building. Let’s say that we wanted to draw a building that is sixteen feet tall using this scale. We could set up a proportion to solve for the number of inches that we would need to draw.

\frac{1\ inch}{4\ feet} = \frac{x\ inches}{16\ feet}

Now we can use what we learned in our last Concept about cross-multiplying to solve proportions. This will help us to figure out the scale dimension of the building.

16 = 4x

Our answer is four inches. The two ratios now form a proportion. What about if we have more than one dimension? Let’s say that we have a room that is 8’ \times 12’ and we want to use a scale of 1” = 2 feet. How many inches long and wide would this drawing be? First, let’s look at the width of the room. It is eight feet wide. We can set up a proportion using the scale and the actual width to figure out the width of the drawing.

\frac{1\ in}{2\ ft} = \frac{x}{8\ ft}

Next, we solve the proportion by cross-multiplying.

2x & = 8 \\x & = 4\ inches

On the drawing, the width will be four inches. Now we need to look at the length. The room is 12 feet long. We can set up a proportion using the scale and the actual length to figure out the length of the drawing.

\frac{1\ in}{2\ ft} = \frac{x}{12\ ft}

Next, we solve the proportion.

2x & = 12 \\x & = 6

On the drawing, the length of the room will be six inches. Next, we can do a scale drawing of this room. If one unit on the drawing is equal to one inch, here is our room.

We can also use scale dimensions to figure out the actual dimensions of something. We will use proportions to do this as well.

The flower bed design shows that the width of the garden on the drawing is six inches. If the scale is 1” = 5 feet, how wide is the actual flower garden? To solve this problem, we need to set up a proportion. Let’s start by writing the scale in the form of a ratio.

\frac{1''}{5\ ft} Next, we can write the actual dimensions that we know with a variable as our unknown and make this the second ratio in this proportion.

\frac{1''}{5'} = \frac{6''}{x}

The drawing of our flower bed is six inches. We can solve the proportion for the actual dimensions of the flower bed by cross-multiplying.

1x & = 30 \\x & = 30\ ft

The actual flower bed is 30 feet wide.

Practice a few of these on your own. Solve each proportion for the scale measurement or the actual measurement.

Example A

\frac{1^{\prime\prime}}{3\ ft} = \frac{x}{21\ ft}

Solution: 7 inches

Example B

\frac{3\ in}{6\ ft} = \frac{9\ in}{x}

Solution: 18 feet

Example C

\frac{2\ in}{10\ ft} = \frac{x}{120\ ft}

Solution: 24 inches

Now let's go back to the problem from the beginning of the Concept.

Jessica is in charge of creating a new project display at the supermarket. Her manager noticed her doodling in a notebook and was impressed with her artistic ability. He called Jessica into his office and handed her a design on a sheet of paper. “We want to create a special poster to focus on a key product each week. This is the sketch of the design. The key product will be on sale and hopefully this new display will help people notice it and buy it,” Jessica’s manager explains.

The design on the page is 8” \times 5”. It is a rectangle and the scale at the bottom of the page says 1” = 6”.

Jessica takes out some foamcore and an exacto knife. She knows that she needs the scale to figure out the exact measurements of the display. The problem is that she can’t remember how to do the math.

“If it were 1” to 1 ft,” she thinks, “Then the poster would be 8 ft \times 5 ft, but that isn’t it. 6" is only 1/2 of 1 ft, so this scale is 1" to 1/2 ft.”

Scale factor is 1” : 6”

Or \frac{1}{6}

The drawing shows that the length of the rectangular sign is 8” and the width is 5”. If the one inch is equal to one half of a foot, then the length is 4 ft and width is 2.5 ft. Jessica is amazed at how simple it actually was to figure that out once she knew how to use the scale factor. Her poster is 4 ft \times 2.5 ft. She gets right to work on the poster and design!

Guided Practice

Here is one for you to try on your own.

\frac{7\ in}{70\ ft} = \frac{x}{140\ ft}

Answer

To find the missing value, we can look at the relationship between the two given denominators. The second denominator is double the first denominator. Because of this, it makes sense that if we double the first numerator that it will give us the value of the second numerator.

Our answer is 14 inches.

Video Review

Khan Academy, Solving Proportions

Explore More

Directions: Find actual dimensions using proportions.

1. The scale of the drawing shows that 1” = 5 feet. If the drawing shows the height of the building as 5 inches, how tall is the actual building?

2.Given this scale, a drawing of a building is 7 inches how tall is the actual building?

3. Given this scale, how tall is a building that has a drawing that is 15 inches?

4. The scale of the drawing shows that 2” = 10 feet. If the drawing shows the height of the building as 8 inches, how tall is the actual building?

5. The scale of the drawing shows that 1” = 3 feet. If the drawing shows the height of the tree as 9 inches, how tall is the tree?

6. The scale of the drawing shows that 2” = 7 feet. If the drawing shows that the height of the tree is 6 inches, how tall is the tree?

7. The scale of the drawing shows that 1” = 3 feet. If the drawing shows that the height of the tree house is 3”, how high is the actual tree house?

Directions: Find the scale dimensions using proportions.

8. The scale of the map shows that 1” = 50 miles. If the map shows that there is 5” between the two cities, what is the actual distance?

9. The scale of the map shows that 2” = 100 km. If the map shows that there are 3” between the two cities, what is the actual distance between them?

10. The scale of the map shows that 4” = 200 km. If the map shows that there are 5 inches between the two cities, what is the actual distance between them?

11. The scale of the garden design shows that 2” = 3 feet. How big is the garden if the rectangular plot is 4” \times 6”?

12. The scale of the room design shows that 1” = 2 feet. How big is the actual room if the design shows a square that is 5 inches wide?

13. The scale of the room design shows 2" = 4 feet. How big is the actual room if the design shows a square that is 10 inches wide?

14. Using this same scale, how big is the actual room if the design shows a square that is 15 inches wide?

15. Using this same scale, how tall is a building if the drawing is 12 inches tall?

Image Attributions

Explore More

Sign in to explore more, including practice questions and solutions for Proportions to Find Dimensions.

Reviews

Please wait...
Please wait...

Original text