crossing path of two rail let us consider following problem
Two trains starting from cities $300$ miles apart head in opposite directions at rates of $70$ mph and $50$ mph, respectively. How long does it take the trains to cross paths?
i would like to  first of all determine geometrical picture of this problem,is it  this one?

here $300$  denoted length between sises?if it is correct then  i dont know exactly how could i solve,suppose that  they cross   after $t$ hour,then first one would have passed $70*t$  second one  $300-50*t$,so equation would be  $70t=300-50*t$,or $120t=300$  from which $t$ is equal to $2.5$ is it correct?

The picture is wrong because the trains are on the same set of tracks, traveling in opposite directions.  There should be no triangle.
Your answer of 2.5 is correct if you say that it is $2.5\ \bf{ hours}$.  Just "$2.5$" by itself is wrong.
