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?

Yes you are correct in calculation .They head in opposite directions means exact opposite
so $$ 70 \times t + 50 \times t = 300 $$
$$ t = 2.5 hours $$
Red guy travels at 70 miles/hr and green guy travels at 50 miles/hr. so they meet at 2.5 hrs at a distance 175 miles from red guys starting place and 125 miles from green guys starting place.
