What is a rate?

• A. The ratio of any two quantities, regardless of units.
• B. A ratio that compares quantities with different units.
• C. The total distance divided by time.
• D. A measure of probability.

Answer: (B)

A rate is a ratio that compares two quantities with different units, expressing “how much of one thing per unit of another.” This is why we talk about speeds as distance per time, or price per item, or mass per volume—it’s about a quantity measured per some unit of another quantity. If you compare two quantities that share the same unit, you end up with a unitless ratio, which isn’t a rate. A rate isn’t about probability; probability measures likelihood. The idea is that a rate uses different units to show how much of one thing happens for each unit of something else, with total distance divided by time serving as a common example of a rate.