Because the quantity you don't know is the dependent variable. That is the variable that depends on all the other quantities. With the dependent variable by itself on one side of the equation, you have everything you don't know on one side and everything you do know on the other side.
Example: If you don't know the distance, but know the rate and time, use . If you don't know the time, but know the rate and distance, use . And if you don't know the rate, but do know the time and distance, then use
John
My calculator said it, I believe it, that settles it
