Question 170683
Lots of ways to look at this one, but the way I prefer is to look at the difference in their two speeds. They are both going the same direction, so the difference between the two is
{{{V = 75 -25}}}
{{{V = 50}}}

How long does it take to go 200km at 50kph?
{{{d = v*t}}}
{{{200 = 50 * t}}}
{{{4 = t}}}
So after 4 hours they are 200 km apart. at the four hour mark, one car is 
{{{d = 75 * 4}}} 
{{{300}}}

The other car is 
{{{d = 25*4}}}
{{{100}}}

SO they are 200 apart