# Proportions

## Lesson Preview

### What You'll Learn

1. To test if ratios can form a proportion
2. To use cross products

### … And Why

To compare answers to a survey, as in Example 1

## Testing Ratios

You know that the ratios and are equal, since each ratio simplifies to .

You can write the equation = .

### Proportion

A proportion is an equation stating that two ratios are equal.

You can test a pair of ratios to determine whether they can form a proportion. One method of testing ratios is to write both ratios in simplest form and see if they are equal.

#### Writing Ratios in Simplest Form

SurveysThere are 24 students in class A and 60 students in class B. Ten students in class A saw a movie this weekend. Twenty-five students in class B saw a movie this weekend. For each class, look at the ratio of the number of students who saw a movie to the total number of students. Can the ratios form a proportion?

Since both ratios are equal to , the ratios are proportional.

Another way to show that two ratios can form a proportion is to show that a common multiplier connects their numerators and denominators.

#### Finding a Common Multiplier

Determine whether the ratios in each pair can form a proportion by finding a common multiplier.

Interactivity Proportions and common multipliers

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