Question 270267
This is a problem of relative speeds 
with 2 trains moving at constant speed
The problem doesn't change at all if 
I say that the train I am trying to pass 
is standing still and I m moving at the
difference in our speeds, {{{100 - 80 = 20}}} km/hr
Now, with the slower train stopped, how long
does it take my front end to go from it's
back end to it's front end, or {{{500}}}m
{{{d = r*t}}}
{{{t = d/r}}}
{{{t = 500/20000}}} (I converted km/hr to m/hr)
{{{t = 5/200}}}
{{{t = 1/40}}} hr
{{{t = (1/40)*60}}} min
{{{t = 1.5}}} min
I passed the 500 m long train at my actual speed of 100000 m/hr, so
{{{d = 100000*(1/40)}}}
{{{d = 10000/4}}}
{{{d = 2500}}}m or
{{{d = 2.5}}}km actual ground covered