Ratios and Rate

Write a ratio as a fraction

When you apply for a mortgage, the loan officer will compare your total debt to your total income to decide if you qualify for the loan. This comparison is called the debt-to-income ratio. A ratio compares two quantities that are measured with the same unit. If we compare $a$ and $b$, the ratio is written $a$ to $b$, $\tfrac{a}{b}$, or $a:b$.

In this section, we will use the fraction notation. When a ratio is written in fraction form, the fraction should be simplified. If it is an improper fraction, we do not change it to a mixed number. Because a ratio compares two quantities, we would leave a ratio as $\tfrac{4}{1}$ instead of simplifying it to $4$, so that we can see the two parts of the ratio.

Example. Write each ratio as a fraction: (a) $15$ to $27$ (b) $45$ to $18$.

(a) Write as a fraction with the first number in the numerator and the second in the denominator: $\tfrac{15}{27}$. Simplify the fraction: $\tfrac{5}{9}$.

(b) Write as a fraction: $\tfrac{45}{18}$. Simplify: $\tfrac{5}{2}$. We leave the ratio in (b) as an improper fraction.

Ratios involving decimals

We will often work with ratios of decimals, especially when we have ratios involving money. In these cases, we can eliminate the decimals by using the Equivalent Fractions Property to convert the ratio to a fraction with whole numbers in the numerator and denominator.

For example, consider the ratio $0.8$ to $0.05$. We can write it as a fraction with decimals and then multiply the numerator and denominator by $100$ to eliminate the decimals:

Do you see a shortcut to find the equivalent fraction? Notice that $0.8 = \tfrac{8}{10}$ and $0.05 = \tfrac{5}{100}$. The least common denominator of $\tfrac{8}{10}$ and $\tfrac{5}{100}$ is $100$. By multiplying the numerator and denominator of $\tfrac{0.8}{0.05}$ by $100$, we “moved” the decimal two places to the right to get the equivalent fraction with no decimals. So we can find the fraction with no decimals by moving both decimal points the same number of places to the right, then simplifying: $\tfrac{80}{5} = \tfrac{16}{1}$.

You do not have to write out every step when you multiply the numerator and denominator by powers of ten. As long as you move both decimal places the same number of places, the ratio will remain the same.

Example. Write each ratio as a fraction of whole numbers: (a) $4.8$ to $11.2$ (b) $2.7$ to $0.54$.

(a) Write as a fraction: $\tfrac{4.8}{11.2}$. Rewrite as an equivalent fraction without decimals, by moving both decimal points $1$ place to the right: $\tfrac{48}{112}$. Simplify: $\tfrac{3}{7}$. So $4.8$ to $11.2$ is equivalent to $\tfrac{3}{7}$.

(b) The numerator has one decimal place and the denominator has two. To clear both decimals we need to move the decimal $2$ places to the right: $\tfrac{2.7}{0.54} = \tfrac{270}{54}$. Simplify: $\tfrac{5}{1}$. So $2.7$ to $0.54$ is equivalent to $\tfrac{5}{1}$.

Some ratios compare two mixed numbers. Remember that to divide mixed numbers, you first rewrite them as improper fractions.

Example. Write the ratio of $1\tfrac{1}{4}$ to $2\tfrac{3}{8}$ as a fraction.

Write as a fraction: $\tfrac{1\frac14}{2\frac38}$. Convert the numerator and denominator to improper fractions: $\tfrac{\frac54}{\frac{19}{8}}$. Rewrite as a division of fractions: $\tfrac{5}{4} \div \tfrac{19}{8}$. Invert the divisor and multiply: $\tfrac{5}{4} \cdot \tfrac{8}{19}$. Simplify: $\tfrac{10}{19}$.

Applications of ratios

One real-world application of ratios that affects many people involves measuring cholesterol in blood. The ratio of total cholesterol to HDL cholesterol is one way doctors assess a person’s overall health. A ratio of less than $5$ to $1$ is considered good.

Example. Hector’s total cholesterol is $249$ mg/dl and his HDL cholesterol is $39$ mg/dl. (a) Find the ratio of his total cholesterol to his HDL cholesterol. (b) Assuming that a ratio less than $5$ to $1$ is considered good, what would you suggest to Hector?

(a) Write as a fraction: $\tfrac{\text{total cholesterol}}{\text{HDL cholesterol}}$. Substitute the values: $\tfrac{249}{39}$. Simplify: $\tfrac{83}{13}$.

(b) Is Hector’s cholesterol ratio OK? If we divide $83$ by $13$ we obtain approximately $6.4$, so $\tfrac{83}{13} \approx \tfrac{6.4}{1}$. Hector’s cholesterol ratio is high! Hector should either lower his total cholesterol or raise his HDL cholesterol.

Ratios of two measurements in different units

To find the ratio of two measurements, we must make sure the quantities have been measured with the same unit. If the measurements are not in the same units, we must first convert them to the same units. We know that to simplify a fraction, we divide out common factors. Similarly in a ratio of measurements, we divide out the common unit.

Example. The Americans with Disabilities Act (ADA) Guidelines for wheelchair ramps require a maximum vertical rise of $1$ inch for every $1$ foot of horizontal run. What is the ratio of the rise to the run?

In a ratio, the measurements must be in the same units. We can change feet to inches, or inches to feet. It is usually easier to convert to the smaller unit, since this avoids introducing more fractions into the problem. Write the ratio as a fraction: $\tfrac{\text{rise}}{\text{run}}$. Substitute in the given values: $\tfrac{1\text{ inch}}{1\text{ foot}}$. Convert $1$ foot to inches: $\tfrac{1\text{ inch}}{12\text{ inches}}$. Simplify, dividing out common factors and units: $\tfrac{1}{12}$.

So the ratio of rise to run is $1$ to $12$. This means that the ramp should rise $1$ inch for every $12$ inches of horizontal run to comply with the guidelines.

Write a rate as a fraction

Frequently we want to compare two different types of measurements, such as miles to gallons. To make this comparison, we use a rate. Examples of rates are $120$ miles in $2$ hours, $160$ words in $4$ minutes, and $\text{\textdollar}5$ dollars per $64$ ounces.

When writing a fraction as a rate, we put the first given amount with its units in the numerator and the second amount with its units in the denominator. When rates are simplified, the units remain in the numerator and denominator.

Example. Bob drove his car $525$ miles in $9$ hours. Write this rate as a fraction.

Write as a fraction, with $525$ miles in the numerator and $9$ hours in the denominator: $\tfrac{525\text{ miles}}{9\text{ hours}} = \tfrac{175\text{ miles}}{3\text{ hours}}$. So $525$ miles in $9$ hours is equivalent to $\tfrac{175\text{ miles}}{3\text{ hours}}$.

Find unit rates

In the last example, we calculated that Bob was driving at a rate of $\tfrac{175\text{ miles}}{3\text{ hours}}$. This tells us that every three hours, Bob will travel $175$ miles. This is correct, but not very useful. We usually want the rate to reflect the number of miles in one hour. A rate that has a denominator of $1$ unit is referred to as a unit rate.

Unit rates are very common in our lives. For example, when we say that we are driving at a speed of $68$ miles per hour we mean that we travel $68$ miles in $1$ hour. We would write this rate as $68$ miles/hour (read “$68$ miles per hour”). The common abbreviation for this is $68$ mph. Note that when no number is written before a unit, it is assumed to be $1$ — so “$68$ miles/hour” really means “$68$ miles/$1$ hour.”

Two rates we often use when driving can be written in different forms:

Another example of unit rate that you may already know about is hourly pay rate. It is usually expressed as the amount of money earned for one hour of work. For example, if you are paid $\text{\textdollar}12.50$ for each hour you work, you could write that your hourly (unit) pay rate is $\text{\textdollar}12.50$/hour (read “$\text{\textdollar}12.50$ per hour”). To convert a rate to a unit rate, we divide the numerator by the denominator. This gives us a denominator of $1$.

Example. Anita was paid $\text{\textdollar}384$ last week for working $32$ hours. What is Anita’s hourly pay rate?

Start with a rate of dollars to hours, then divide. Write as a rate:

Divide the numerator by the denominator to get the unit rate:

Rewrite as a rate: $\text{\textdollar}12$/hour. Anita’s hourly pay rate is $\text{\textdollar}12$ per hour.

Example. Sven drives his car $455$ miles, using $14$ gallons of gasoline. How many miles per gallon does his car get?

Start with a rate of miles to gallons, then divide: write as a rate, $\tfrac{455\text{ miles}}{14\text{ gallons}}$; divide $455$ by $14$ to get the unit rate, $\tfrac{32.5\text{ miles}}{1\text{ gallon}}$. Sven’s car gets $32.5$ miles/gallon, or $32.5$ mpg.

Find unit price

Sometimes we buy common household items “in bulk,” where several items are packaged together and sold for one price. To compare the prices of different sized packages, we need to find the unit price. To find the unit price, divide the total price by the number of items. A unit price is a unit rate for one item.

Example. The grocery store charges $\text{\textdollar}3.99$ for a case of $24$ bottles of water. What is the unit price?

We are asked to find the unit price, which is the price per bottle. Write as a rate:

Divide to find the unit price:

Round the result to the nearest penny:

The unit price is approximately $\text{\textdollar}0.17$ per bottle.

Unit prices are very useful if you comparison shop. The better buy is the item with the lower unit price. Most grocery stores list the unit price of each item on the shelves.

Example. Paul is shopping for laundry detergent. At the grocery store, the liquid detergent is priced at $\text{\textdollar}14.99$ for $64$ loads of laundry and the same brand of powder detergent is priced at $\text{\textdollar}15.99$ for $80$ loads. Which detergent has the lowest cost per load?

To compare the prices, we first find the unit price for each type of detergent:

Now we compare the unit prices. The unit price of the liquid detergent is about $0.23 per load and the unit price of the powder detergent is about $0.20 per load. The powder is the better buy.

Notice that we rounded the unit price to the nearest cent. Sometimes we may need to carry the division to one more place to see the difference between unit prices.

Translate phrases to expressions with fractions

Have you noticed that the examples in this section used the comparison words ratio of, to, per, in, for, on, and from? When you translate phrases that include these words, you should think either ratio or rate. If the units measure the same quantity (length, time, etc.), you have a ratio. If the units are different, you have a rate. In both cases, you write a fraction.

Example. Translate the word phrase into an algebraic expression: (a) $427$ miles per $h$ hours (b) $x$ students to $3$ teachers (c) $y$ dollars for $18$ hours.

(a) Write as a rate: $\tfrac{427\text{ miles}}{h\text{ hours}}$.

(b) Write as a rate: $\tfrac{x\text{ students}}{3\text{ teachers}}$.

(c) Write as a rate: $\tfrac{y\text{ dollars}}{18\text{ hours}}$.

Key terms

ratio — a comparison of two numbers or quantities measured with the same unit, written $a$ to $b$, $\tfrac{a}{b}$, or $a:b$. rate — a comparison of two quantities measured in different units, usually written as a fraction. unit rate — a rate with a denominator of $1$ unit. unit price — a unit rate that gives the price of one item.

This section is adapted from Prealgebra 2e, Section 5.6: Ratios and Rate by Lynn Marecek, MaryAnne Anthony-Smith, and Andrea Honeycutt Mathis, © OpenStax, licensed under CC BY-NC-SA 4.0. Access the original for free at openstax.org. Changes: recreated the driving-rate and detergent-comparison tables as markdown tables; omitted the Be Prepared quiz, Media links, and end-of-section exercises; and converted the practice problems (“Try Its”) into interactive exercises with instant feedback.