Question 390238
You can think of one train as standing still
and the other train moving at the sum of
their speeds
The formula for the moving train is
{{{d = r*t}}}
{{{d = 780}}} km
{{{r = 120 + 140}}}
{{{r = 260}}}
{{{780 =  260*t}}}
{{{t = 3}}} hrs
The trains will pass at 3 PM + 3 hrs, or 6 pm
You can also check answer this way:
slower train:
{{{d[1] = r*t}}}
{{{d[1] = 120*3}}}
{{{d[1] = 360}}} km
faster train:
{{{d[2] = 140*3}}}
{{{d[2] = 420}}}
{{{d[1] + d[2] = 360 + 420}}}
{{{d[1] + d[2] = 780}}} km
OK