Question 835077
_____________speed____________time_____distance_______
Faster_______r_________________6_______r*6____________
Slower______r-15_______________6_______(r-15)*6_______
Total__________________________________639


Last part of the description and the distance column give the equation,
{{{highlight(r*6+(r-15)*6=639)}}}
-
{{{6r+6r-90=639}}}
{{{12r=639+90}}}
{{{12r=729}}}
{{{highlight(r=61)}}} mph



(Actual computation to r=60.75)