The further the distance, the longer particle lifetime. How?

Guys, I've emitted particles from object to target. But the particles doesn't reach the further targets and the particles linger around too long for nearer targets.

Since this is a 2D game, I'm guessing X-axis is enough to determine its distance, but I don't know how to ratio it to the particle's lifetime.

Is there a Mathf equation for ratio calculations?

or if I were to be more specific(just a Mathf equation would be a great help already) Let's say at 100 X-distance, the lifetime of the particle is 0.5f. Then the further away the target is, the longer the lifetime.

Anyone have any Mathf calculations? Thanks in advance!