Question 837564
Here will be shown help to start, but not the whole solution.


Key idea is R*T=D, R for rate, T for time, D for distance.


First 80 miles at the speed, r.
Last 105 miles at the speed r-5.


_______________speed_________time_________distance
First___________r____________(____)________80
Last___________r-5___________(____)________105
TOTAL________________________5


You need to fill the information for the two time quantities.  You know that the equation ,  r*t=80, so you can solve this for t, and that gives the expression for the First time.  You understand what to do for the "Last" time?  

Can you continue and finish this?