Question 1107071
<pre>
                    speed      time         distance
part1                r         200/r         200
part2               r-20       150/(r-20)    150
total                          5
</pre>

{{{200/r+150/(r-20)=5}}}
Simplify and solve this for r.

{{{40/r+30/(r-20)=1}}}

{{{40(r-20)+30r=r^2-20r}}}

{{{40r-800+30r=r^2-20r}}}

{{{r^2-20r-70r-800=0}}}

{{{r^2-90r-800=0}}}

{{{r=(90+- sqrt(8100+4*800))/2}}}

{{{r=(90+- sqrt(4900))/2}}}

{{{r=(90+- 70)/2}}}

{{{r=(45+- 35)}}}

{{{cross(r=5)}}} or {{{highlight(r=80)}}}

The 80 km per hour is the one which makes sense.