Definition: A rate is a ratio that compares two quantities measured in different units, such as miles per hour, dollars per pound, or words per minute.
What makes a ratio a rate?
When the two quantities have different units, the ratio is called a rate. Mathbot recharges 45 percent of his battery in 30 minutes. The units, percent and minutes, differ, so 45 percent to 30 minutes is a rate.
How do you write a rate?
Use the word per or a fraction bar: 45 percent per 30 minutes, or 45/30 with units attached. You may simplify, just as with a ratio. Dividing both numbers by 15 gives 3 percent per 2 minutes.
Why do the units matter?
The units tell you what the numbers mean. A rate of 240 miles in 4 hours is not the same message as 240 miles in 4 gallons. Always keep the units next to the numbers, and say which quantity comes first.
What should you watch for?
Different rates can describe the same situation. A walker covering 6 miles in 2 hours is also going 3 miles in 1 hour, or 1 mile in 20 minutes. Pick the form that answers the question you were asked.