Question 890692
SEE THE "BETTER" SECTION BELOW!


The time for the trip of d is unknown t in hours.
15 minutes is {{{1/4}}} hour.


___________________speed________time_______distance
One speed___________x____________t___________d
Other speed_________x+5_________t-(1/4)______d


You cannot completely answer the question because you have three unknown variables and at best, two equations.
A symbolic solution is still possible.


{{{xt=d}}} and {{{(x+5)(t-1/4)=d}}}.
{{{xt-x/4+5t-5/4=xt}}}
{{{-x/4+5t-5/4=0}}}



{{{-x/4=5/4-5t}}}
{{{x=-5+20t}}}
{{{highlight(x=20t-5)}}}


-----------MISUNDERSTOOD.

I now see, "25 km" so the distance is really known.  You can actually solve both for t and for x.


------------BETTER-------------------------
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___________________speed________time_______distance
One speed___________x____________t___________25
Other speed_________x+5_________t-(1/4)______25


{{{highlight_green(xt=25)}}} and {{{(x+5)(t-1/4)=25}}}.
The second equation------
{{{xt+5t-x/4-5/4=25}}}
{{{25+5t-x/4-5/4=25}}}
{{{5t-x/4-5/4=0}}}
{{{highlight_green(20t-x-5=0)}}}
We do not care so much about t, so use the first equation to find how to substitute.
{{{20t=x+5}}}
{{{t=(x+5)/20}}}
Substitution,
{{{xt=25}}}
{{{x(x+5)/20=25}}}
{{{x(x+5)=20*25}}}
{{{x^2+5x=500}}}
{{{highlight(x^2+5x-500)}}}   ----------SOLVE THIS FOR x.