Question 640960
I think of one train as standing still and the other
traveling at the sum of their speeds
{{{ 780 = ( 120 + 140 )*t }}}
{{{ 780 = 260t }}}
{{{ t = 3 }}} 
3 PM + 3 hrs is 6 PM
check:
In 3 hrs, slower train travels
{{{ 120*3 = 360 }}} km
faster train travels
{{{ 140*3 = 420 }}} km
{{{ 360 + 420 = 780 }}} km
OK