The speed of an object is equal to the distance traveled divided by the time. This is equivalent to the statement that as the time taken to travel a distance increases, the speed of the object must decrease.
The standard unit for speed in science is meters per second. If, for example, a person runs 30 meters in 10 seconds, his speed is 3 meters per second. The relationship for speed is the same for any units, as long as the units are consistent. If a car traveled 30 miles in an hour, the speed would be 30 miles per hour.
If an object is moving at varying speeds, then the relationship between speed, distance and time can only be used to calculate the average speed. A distance-time graph, which shows the speed varying as the time increases, is a good way to see how the speed of an object changes.
The relationship between speed, distance and time can be used to calculate any of the three variables, as long as the other two are known. The time taken for a journey, for example, is equal to the distance divided by the speed. Distance traveled can be calculated by multiplying speed and time together.