S............a...............[you].......a-20.........F
a + a-20 = 300
Didn't understand what you meant at first, I see you mean you are 20 mi closer to the finish line than you are to the start line.
call your distance to start line x
then your distance to finish line is 300-x
we have 300-x = x-20
that is, your (distance to finish line) equals (distance to start line) minus 20.
solve for x.