Question 876942
You have a description for speeds but no sense of direction.  


{{{r*3+(r+5)*3=393}}}
{{{3r+3r+5=393}}}
{{{6r=393-5}}}
{{{6r=388}}}
{{{r=64&2/3}}} mph, the slower car.



{{{(r+5)*3-r*3=393}}}
{{{3r+15-3r=393}}}
{{{15=393}}}, nonsense.


The cars could be going in opposite directions.