Question 459622
First car  speed = x kmph
Second car speed = y kmph
..
second car passes the lad mark 1/2 hour later.
So in Half hour Car A has passes = x/2 km
..
They are travelling in same direction
so catch up speed = y-x kmph
..
Catch up distance = x/2 km
,...
Catch up time = (x/2) / (y-x) hours 
=x/(2(y-x)hours