Problem Solving Plan,
Proportions
Jen Kershaw
Say Thanks to the Authors
Click http://www.ck12.org/saythanks
(No sign in required)
To access a customizable version of this book, as well as other
interactive content, visit www.ck12.org
CK-12 Foundation is a non-profit organization with a mission to
reduce the cost of textbook materials for the K-12 market both
in the U.S. and worldwide. Using an open-content, web-based
collaborative model termed the FlexBook®, CK-12 intends to
pioneer the generation and distribution of high-quality educational
content that will serve both as core text as well as provide an
adaptive environment for learning, powered through the FlexBook
Platform®.
Copyright © 2014 CK-12 Foundation, www.ck12.org
The names “CK-12” and “CK12” and associated logos and the
terms “FlexBook®” and “FlexBook Platform®” (collectively
“CK-12 Marks”) are trademarks and service marks of CK-12
Foundation and are protected by federal, state, and international
laws.
Any form of reproduction of this book in any format or medium,
in whole or in sections must include the referral attribution link
http://www.ck12.org/saythanks (placed in a visible location) in
addition to the following terms.
Except as otherwise noted, all CK-12 Content (including CK-12
Curriculum Material) is made available to Users in accordance
with the Creative Commons Attribution-Non-Commercial 3.0
Unported (CC BY-NC 3.0) License (http://creativecommons.org/
licenses/by-nc/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated
herein by this reference.
Complete terms can be found at http://www.ck12.org/terms.
Printed: October 10, 2014
AUTHOR
Jen Kershaw
www.ck12.org
C HAPTER
Chapter 1. Problem Solving Plan, Proportions
1
Problem Solving Plan,
Proportions
Here you’ll learn to use the problem solving strategy: use a proportion.
Have you ever studied frogs?
Tim loves to read about frogs. While his Mom was grocery shopping, Tim caught sight of a magazine all about
frogs. He couldn’t help picking it up and was glad that he had a few dollars in his pocket to buy the magazine. When
Tim got to the car, he was amazed to read that a frog can jump twenty times its body length. That means if a frog is
three inches long, it can jump 20 times that far: 5 feet!
3 × 20 = 60 inches or 5 feet
“Mom, listen to this,” Tim exclaimed, as he shared his findings.
“Wow Tim, I didn’t know that. You are four feet tall, how far could you jump if you could jump like a frog?” Tim
stopped to think. He wasn’t sure he knew how to figure that out, but he was sure that proportions would be a part of
it.
Do you know how to figure this out? Use the information in this Concept to help Tim hop like a frog.
Guidance
This Concept focuses on using a proportion to solve a problem. To use a proportion to solve a problem certain
criteria must be present. If these criteria are not present, you can’t use a proportion to solve the problem.
What information needs to be present in a problem to solve it using a proportion?
To use a proportion in a problem, the problem must have information that is being compared. It must also
have the same information in both groups. If you think about this it makes perfect sense. A proportion compares
two equal ratios-if the information in the problem is different, then two different things are being compared and they
are not equal.
A cheetah can run 75 miles per hour. If you could run three times as fast as a cheetah, how fast would you be able to
run?
1
www.ck12.org
This problem compares the same quantities. It compares the cheetah’s speed per hour and the person’s speed
per hour. Here is a proportion showing the comparison.
cheetah0 s speed
Person0 s speed
=
number o f hours number o f hours
When you look at these two comparisons, you can see that we are comparing speed with speed. We can use a
proportion to solve this problem.
Sometimes you have a problem where you can’t use a proportion.
A car travels 55 miles in two hours. A bus travels 85 kilometers in two hours. Which vehicle traveled a farther
distance?
In this problem our units are not the same. We are comparing hours with hours, but we are comparing
miles with kilometers, so the units are different. We could not use a proportion to solve this problem without
converting the units first.
Let’s apply a proportion and use it to solve the cheetah problem.
A cheetah can run 75 miles per hour. If you could run three times as fast as a cheetah, how fast would you be able to
run?
We already wrote the proportion to show what is being compared in this problem. Here is the proportion.
cheetah0 s speed
Person0 s speed
=
number o f hours number o f hours
Our next step is to take the data and fill it into the proportion.
75 x
=
1
3
Here we wrote in that the cheetah runs 75 miles per hour. Per means "divided by", and "hour" refers to only one, so
we use one as our denominator and 75 as the speed in the numerator. The person runs three times as fast, so he or
she would go as far in 1 hour as a cheetah would in 3 hours. We put 3 in for the denominator. We multiplied the
denominator by 3, so the numerator becomes 3(75), and we will use a variable for the person’s speed because we
don’t know what it is yet.
Next we solve the proportion using cross products.
x = 75(3)
x = 225 mph
2
www.ck12.org
Chapter 1. Problem Solving Plan, Proportions
If a person ran three times as fast as a cheetah, he or she would run 225 mph. That is very fast indeed!!
Now you can try a few on your own. Solve each problem by using proportions.
Example A
If you could run twice as fast as a cheetah, how fast could you run?
Solution: 150 mph
Example B
If you could run half as fast as a cheetah, how fast could you run?
Solution: 37.5 mph
Example C
If you could run four times as fast as a cheetah, how fast could you run?
Solution: 300 mph
Ready? Using what you have just learned, you should be ready to help Tim figure out his frog dilemma. Here is the
problem once again.
Tim loves to read about frogs. While his Mom was grocery shopping, Tim caught sight of a magazine all about
frogs. He couldn’t help picking it up and was glad that he had a few dollars in his pocket to buy the magazine. When
Tim got to the car, he was amazed to read that a frog can jump twenty times its body length. That means if a frog is
three inches long, it can jump 20 times that far: 5 feet!
3 × 20 = 60 inches or 5 feet
“Mom, listen to this,” Tim exclaimed, as he shared his findings.
“Wow Tim, I didn’t know that. You are four feet tall, how far could you jump if you could jump like a frog?”
3
www.ck12.org
Tim stopped to think. He wasn’t sure he knew how to figure this out, but he was sure that proportions would be a
part of it.
To start with, let’s write a proportion to compare the frog’s length and its jump to Tim’s height and his jump.
f rog length Tim0 s height
=
f rog jump
Tim0 s jump
Now that we have the proportion, we can fill in the information that we know.
300
4ft
=
00
60
x
Oh, here is our first problem. The frog is in inches and Tim’s height is in feet. Let’s change 4 ft to inches.
4 × 12 = 48”
Now we can solve for the variable which is how far Tim will jump. We do this by solving the proportion.
4800
300
=
00
60
x
3x = 2880
x = 96000
We divided 2880 by three and got 960 inches as our answer. Now we can convert this to feet to show how far
Tim jumped.
80
12)960
If Tim were a frog he could jump 80 feet. If you enjoyed this problem, check out “If You Hopped Like a Frog”
by David M. Schwartz-a great picture book of proportions.
Vocabulary
Here is a vocabulary word in this Concept.
Proportion
an equal comparison of two ratios.
Guided Practice
Here is one for you to try on your own.
Use a proportion to solve the following problem.
If a person can run 3 miles in 20 minutes, how long will it take the same person to run 12 miles if it is at the same
rate?
Answer
4
www.ck12.org
Chapter 1. Problem Solving Plan, Proportions
In this problem, we are comparing miles and time. That is our ratio. Let’s set it up.
miles
time
=
miles
time
Next we fill in the given information.
3
20
=
12
x
Now we cross multiply and solve.
3x = 240
x = 80
The person would run 12 miles in 80 minutes.
Video Review
Here are videos for review.
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5431
Khan Academy, Understanding Proportions
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/5418
James Sousa, Applications Using Proportions
Practice
Directions: Solve each word problem by using a proportion.
1. In a diagram for the new garden, one inch is equal to 3 feet. If this is the case, how many feet is the actual garden
edge if the measurement on the diagram is 5 inches?
2. If two inches on a map are equal to three miles, how many miles are represented by four inches?
3. If eight inches on a map are equal to ten miles, how many miles are 16 inches equal to?
4. Casey drew a design for bedroom. On the picture, she used one inch to represent five feet. If her bedroom wall is
ten feet long, how many inches will Casey draw on her diagram to represent this measurement?
5. If two inches are equal to twelve feet, how many inches would be equal to 36 feet?
6. If four inches are equal to sixteen feet, how many feet are two inches equal to?
5
www.ck12.org
7. The carpenter chose a scale of 6” for every twelve feet. Given this measurement, how many feet would be
represented by 3”?
8. If 9 inches are equal to 27 feet, how many feet are equal to three inches?
9. If four inches are equal to 8 feet, how many feet are equal to two inches?
10. If six inches are equal to ten feet, how many inches are five feet equal to?
11. If four inches are equal to twelve feet, how many inches are equal to six feet?
12. For every 20 feet of fence, John drew 10 inches on his plan. If the real fence is only 5 feet long, how many
inches will John draw on his plan?
13. If eight inches are equal to twelve feet, how many inches are equal to six feet?
14. How many inches are equal to 20 feet if 4 inches are equal to 10 feet?
15. How many inches are equal to 8 feet if six inches are equal to 16 feet?
16. Nine inches are equal to twelve feet, so how many inches are equal to 4 feet?
17. If a person runs two miles in twelve minutes, how long will it take them to run 4 miles at the same rate?
18. A person runs 1 mile in 16 minutes. Given this information, how long will it take him/her to run 3 miles?
19. If a person runs two miles in twenty minutes, at what rate does he/she run one mile?
6