When you talk about the speed of a car, you usually say something in miles per hour. For example, you say, 'I drove 40 miles per hour.' Normally, you don't say, 'I drove 120 miles per 3 hours.' Figure out how to convert a rate like 120 miles per 3 hours to the unit rate of 40 miles per hour by watching this tutorial.
Can you do 100 sit-ups in 2 minutes? That's a rate! Driving a car going 40 miles per hour? That's a unit rate! Watch this tutorial to learn about rate and unit rate (and the difference!).