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In these lessons, we will learn how to solve direct proportions (variations) and inverse proportions (inverse variations) problems. (Note: Some texts may refer to inverse proportions/variations as indirect proportions/variations.)

Related Topics:

Direct Variations, Proportion Word Problems, More Algebra Lessons

Two values x and y are

Knowing that the ratio does not change allows you to form an equation to find the value of an unknown variable.

Example:If two pencils cost $1.50, how many pencils can you buy with $9.00?

Solution:

The number of pencils is directly proportional to the cost.

pencils

This video shows how to solve direct proportion questions

Example 1: F is directly proportional to x. When F is 6, x is 4. Find the value of F when x is 5.

Example 2: A is directly proportional to the square of B. When A is 10, B is 2. Find the value of A when B is 3.

This video shows how to solve word problems by writing a proportion and solving

1. A recipe uses 5 cups of flour for every 2 cups of sugar. If I want to make a recipe using 8 cups of flour, how much sugar do I use?

2. A syrup is made by dissolving 2 cups of sugar in 2/3 cups of boiling water. How many cups of sugar should be used for 2 cups of boiling water?

3. A school buys 8 gallons of juice for 100 kids. how many gallons do they need for 175 kids?

1. On a map, two cities are 2 5/8 inches apart. If 3/8 inches on the map represents 25 miles, how far apart are the cities (in miles)?

2. Solve for the sides of similar triangles using proportions

Two values x and y are

Knowing that the product does not change also allows you to form an equation
to find the value of an unknown variable

Example:

It takes 4 men 6 hours to repair a road. How long will it take 8 men to
do the job if they work at the same rate?

Solution:

The number of men is inversely proportional to the time taken to do the job.

Let t be the time taken for the 8 men to finish the job.

4 × 6 = 8 × t

24 = 8t

t = 3 hours

Usually, you will be able to decide from the question whether the values are
directly proportional or inversely proportional.

This video shows how to solve inverse proportion questions. It goes through a couple of examples and ends with some practice questions

Example 1: A is inversely proportional to B. When A is 10, B is 2. Find the value of A when B is 8

Example 2: F is inversely proportional to the square of x. When A is 20, B is 3. Find the value of F when x is 5.

How to use a more advanced form of inverse proportion where the use of square numbers is involved.

In math, an inverse proportion is when an increase in one quantity results in a decrease in another quantity. This video will show how to solve an inverse proportion math problem.

You can use the Mathway widget below to practice Algebra or other math topics. Try the given examples, or type in your own problem. Then click "Answer" to check your answer.