Unit Rate Definition: Lesson for Kids

Instructor: Jenny Homer

Jenny has masters' degrees in public health and public administration.

Unit rates might sound like a new math concept, but they're actually something you've probably come across many times without even knowing it! Learn about unit rates in the lesson that follows.

How Fast Can This Car Go?

Have you ever been riding in a car and seen a sign on the side of the street showing the speed limit? The speed limit tells us how fast we are allowed to drive, and the sign usually has one big number on it.

Speed limit sign
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You may be surprised to know that the speed limit isn't just any ordinary number. If the sign says 45, it's actually telling us that 45 miles per hour (mph) is the top speed we're allowed to drive. In math, this is called a unit rate. In this lesson, we'll first talk about rates in general, and then we'll focus on a unit rate to understand what ''miles per hour'' really means.

Understanding Rates

Let's say that you are going to make a batch of slime. You'll need 8 ounces of glue and 4 ounces of water. When we want to compare one thing to another, we use a ratio. In this case, the ratio of glue to water is 8:4. This can also be written as 8 to 4, or as the fraction 8/4.

With ratios, you can multiply or divide the top and bottom number by the same thing. Say your friend is coming over and you want to make two batches of slime. If you multiply both numbers in your ratio by 2, you'll see that you need a ratio of glue to water that is 16:8. Or if one batch makes way too much, you can divide both numbers of the original ratio by 2, and have a ratio of 4:2.

A rate is one type of ratio that compares two different kinds of measures. With the slime, both parts of the ratio are ounces of a liquid, but with mph we're using miles to measure distance and hours to measure time.

Rates are used for many different purposes, like when a doctor measures a patient's heart rate in number of beats per minutes or when we buy a bunch of grapes and pay a certain dollar amount per pound.

The price of grapes at stores is usually per pound.
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Calculating Unit Rates

When the second number of the rate or the denominator (the bottom number of the fraction) is 1, the rate is a unit rate. Mph is a unit rate because it tells us the speed of traveling a certain number of miles in 1 hour.

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