a variable (often denoted by x ) whose variation does not depend on that of another.
a variable (often denoted by y ) whose value depends on that of another.
This is for math of corse but we can use this to answer it, all you have to do is find the x which is time, then y is just the opposite of time, hope this helps. Let me know if you have any questions or want me to help you on a question I know my answer was a bit veg.
