<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Dismiss
Skip Navigation

5.10: Proportions Using Cross Products

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
Estimated6 minsto complete
%
Progress
Practice Proportions Using Cross Products
 
 
 
MEMORY METER
This indicates how strong in your memory this concept is
Practice
Progress
Estimated6 minsto complete
%
Estimated6 minsto complete
%
Practice Now
MEMORY METER
This indicates how strong in your memory this concept is
Turn In

Jamaya dropped bottles and wrappers whenever and wherever she wanted to. In order to break this bad habit, Jamaya’s mother made her help with a local clean-up effort. For every piece of litter that Jamaya had dropped in and around her home, she had to pick up 10 pieces with the group. If Jamaya dropped a total of 15 bottles, cans, and wrappers, how many pieces of litter does she have to clean up?

In this concept, you will learn how to solve proportions using cross products.

Solving Proportions Using Cross Products

ratio represents a comparison between two quantities. Equivalent ratios are ratios that are equal. A proportion is made up of two equivalent ratios.

Proportional reasoning, or examining the relationship between two numbers, can be used to determine the value of x.

A proportion can be expressed as two equivalent fractions.

 \begin{align*}\frac{a}{b}=\frac{c}{d}\end{align*}

A proportion can be expressed with colons.

\begin{align*}a:b = c:d\end{align*}

In a proportion, the means are the two terms that are closest together when the proportion is written with colons. So, in \begin{align*}a:b = c:d\end{align*}, the means are \begin{align*}b\end{align*} and \begin{align*}c\end{align*}.

The extremes are the terms in the proportion that are furthest apart when the proportion is written with colons. So, in \begin{align*}a:b = c:d\end{align*}, the extremes are \begin{align*}a\end{align*}and \begin{align*}d\end{align*}.

The diagram below shows how to identify the means and the extremes in a proportion.

The Cross Products Property of Proportions states that the product of the means is equal to the product of the extremes.

 \begin{align*}\frac{a}{b}&=\frac{c}{d}\\ b\cdot c&=a\cdot d\end{align*}

Here is an example.

Solve for a.

\begin{align*}\frac{a}{4}=\frac{6}{8}\end{align*}

First, multiply the means and the extremes and set them equal to one another.

\begin{align*}a\cdot 8&=4\cdot6\\ 8a&=24\end{align*}

Next, solve the equation for the missing variable. 

\begin{align*}\frac{8a}{8}&=\frac{24}{8}\\ a&=3\end{align*}

The answer is a = 3.

Examples

Example 1

Earlier, you were given a problem about Jamaya and her littering habit.

She had dropped a total of 15 pieces of litter in and around her home, and her mother said she had to pick up 10 pieces for every piece that she dropped.   How many pieces of litter does Jamaya have to clean up?

First, write a proportion.

 \begin{align*}\frac{10 \ pieces \ to \ pick \ up }{1 \ piece \ dropped}= \frac{x \ pieces \ to \ pick \ up}{15 \ pieces \ dropped}\end{align*}

Next, cross multiply.

\begin{align*}x = 150\end{align*}

The answer is x=150.

Jamaya must pick up 150 pieces of litter.

Example 2

Solve for x.

\begin{align*}\frac{x}{5}=\frac{6}{10}\end{align*}

First, cross multiply and set the products equal to one another.

\begin{align*}10x = 30\end{align*}

Next, solve for x.

\begin{align*}x = 3\end{align*}

The answer is x=3.

Example 3

Solve for a.

\begin{align*}\frac{a}{9}=\frac{15}{27}\end{align*}

First, cross multiply and set the products equal to one another.

\begin{align*}27a = 135\end{align*}

Next, solve for a.

\begin{align*}a = 5\end{align*}

The answer is a = 5.

Example 4

Solve for b.

\begin{align*}\frac{b}{4}=\frac{12}{16}\end{align*}

First, cross multiply and set the products equal to one another.

\begin{align*}16b = 48\end{align*}

Next, solve for b.

\begin{align*}b = 3\end{align*}

The answer is b = 3.

Example 5

Rudy was a silly kitten who loved to play with ping pong balls. It took him just 15 minutes to swat 12 of them underneath the sofa. How many ping pong balls could Rudy hit under the sofa in 1 hour?

First, write a proportion.

 \begin{align*}\frac{15 \ minutes}{12 \ balls} = \frac{60 \ minutes}{x \ balls}\end{align*}

Next, cross multiply.

\begin{align*}15x = 720\end{align*}

Then, solve for x.

\begin{align*}x = 48\end{align*}

The answer is 48. In one hour, 60 minutes, Rudy could swat 48 ping pong balls under the sofa.

Review

Use cross products to find the value of the variable in each proportion.

  1. \begin{align*}\frac{6}{10} = \frac{x} {5}\end{align*}
  2. \begin{align*}\frac{2}{3} = \frac{x} {9}\end{align*}
  3. \begin{align*}\frac{4}{9} = \frac{a} {45}\end{align*}
  4. \begin{align*}\frac{7}{8} = \frac{a} {4}\end{align*}
  5. \begin{align*}\frac{b}{8} = \frac{5} {16}\end{align*}
  6. \begin{align*}\frac{6}{3} = \frac{x} {9}\end{align*}
  7. \begin{align*}\frac{4}{x} = \frac{8} {10}\end{align*}
  8. \begin{align*}\frac{1.5}{y} = \frac{3} {9}\end{align*}
  9. \begin{align*}\frac{4}{11} = \frac{c} {33}\end{align*}
  10. \begin{align*}\frac{2}{6} = \frac{5} {y}\end{align*}
  11. \begin{align*}\frac{2}{10} = \frac{5} {x}\end{align*}
  12. \begin{align*}\frac{4}{12} = \frac{6} {n}\end{align*}
  13. \begin{align*}\frac{5}{r} = \frac{70} {126}\end{align*}
  14. \begin{align*}\frac{4}{14} = \frac{14} {k}\end{align*}
  15. \begin{align*}\frac{8}{w} = \frac{6} {3}\end{align*}
  16. \begin{align*}\frac{2}{5} = \frac{17} {a}\end{align*}

Review (Answers)

To see the Review answers, open this PDF file and look for section 5.10.

Resources

 

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Please to create your own Highlights / Notes
Show More

Vocabulary

Cross Product Property of Proportions

The cross product property of proportions states that the cross products of two ratios will be equal if the two ratios form a proportion.

Cross Products

To simplify a proportion using cross products, multiply the diagonals of each ratio.

Extremes

In a proportion, the extremes are the values of the proportion that are furthest apart when written in ratio form using a colon. For example: In the proportion a : b = c : d, a and d are the extremes.

Means

In a proportion, the means are the values of the proportion that are close to each other when written in ratio form using a colon.

Image Attributions

Show Hide Details
Description
Difficulty Level:
At Grade
Grades:
Date Created:
Dec 21, 2012
Last Modified:
Sep 08, 2016
Save or share your relevant files like activites, homework and worksheet.
To add resources, you must be the owner of the Modality. Click Customize to make your own copy.
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original
 
MAT.ARI.631.L.2
Here