Scientists use light years to calculate distance because they can tell how far away things are in space by measuring how much time it takes for light to reach a certain point. It's just easier to say how many light years things are in space then to say in miles.
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Speed = distance divided by time
Speed = Distance divided by Time
You can calculate a speed by dividing a distance by the time it takes to cover that distance. If you want the instantaneous speed (for situations of variable speed), you need to calculate the distance and time for a fairly short time interval (ideally, the limit, when the time approaches zero).
It's the fastest speed anyone can prove exists. Any speeds in excess of light speed are still just theory. Some scientists believe it is the fastest speed that will ever be achievable.( a cosmic speed limit.)
Average speed = (distance covered) divided by (time to cover the distance)