Question 266105
By far, the easiest way to solve this is to just visualize it
and don't think of it as 2 cars, but 1 car which is going at 
the combined speed of both cars. You don't care where that 1 
car starts from. All you care about is that it goes {{{420}}} mi
in {{{4}}} hrs. Call the speed {{{s + s + 5}}}, the combined speed.
The equation would be:
{{{420 = (s + s + 5)*4}}}
{{{420 = 8s + 20}}}
{{{8s = 400}}}
{{{s = 50}}} mi/hr
{{{s + 5 = 55}}} mi/hr
The faster car goes 55 mi/hr and
the slower car goes 50 mi/hr
check answers:
{{{d[1] = 55*4}}}
{{{d[1] = 220}}} mi
{{{d[2] = 50*4}}}
{{{d[2] = 200}}} mi
{{{200 + 220 = 420}}}
OK