Question 724611
Let r = the unknown usual speed.


______________rate______________time___________________distance
Usual_________r_________________t______________________660
Faster_______r+90______________t-7____________________660


We want to solve for r, but t is also a variable and unknown.  Fortunately, the rows of data use the SAME distance.  We can solve for the times in terms of distance and rates.  rate*time=distance.


t=660/r, so t-7=660/r-7.


______________rate______________time___________________distance
Usual_________r_________________660/r______________________660
Faster_______r+90______________660/r-7____________________660

The "Faster" data may be of use.  {{{(r+90)*(660/r-7)=660}}}


... WAIT!
... Interrupting that, there are TWO equations possible:
{{{rt=660}}}
{{{(r+90)(t-7)=660}}}
Going ahead with that system, the second equation becomes
{{{rt-7r+90t-630=660}}}
{{{rt+90t-7r=30}}}


and from the first equation, find that t=660/r and substitute to get
{{{r(660/r)+90(660/r)-7r=30}}}
{{{660+90*660/r-7r=30}}}
{{{660r+90*660-7r^2=30}}}
{{{-7r^2+660r+90*660-30=0}}}
{{{-7r^2+660r+59370=0}}}
{{{highlight(7r^2-660r-59370=0)}}}
Using general solution to quadratic equation, {{{highlight(r=150)}}} km/hr


I have not yet tried to check that solution.