Calculus
Some examples of variables are distance and time, because they can change. The speed of an object is how far it travels in a particular time. So if a town is 80 kilometres (50 miles) away and a person in a car gets there in one hour, they have traveled at an average speed of 80 kilometres (50 miles) per hour. But this is only an average: they travelled faster at some times (say on a highway), and slower at other times (say at a traffic light or on a small street where people live). Certainly it is more difficult for a driver to figure out a car's speed using only its odometer (distance meter) and clock—without a speedometer.