Question 259134
Your average speed on the first half of a car trip is 70.0 km/h. How fast do you have to drive on the second half of the trip to achieve an average speed of 85.0 km/h for the whole trip?
<pre><font size = 4 color = "indigo"><b>
Let's suppose the whole trip is 2D km, since time = distance/speed, you
have to get there in {{{2D/85}}} hours.  You have gone D km at 70 miles
an hour so again since time = distance/speed you have used up {{{D/70}}}
hours of the {{{2D/85}}} hours, and you have only {{{2D/85-D/70}}} hours
left to go the remaining D km.  Since speed = distance/time, the

speed = {{{D/(2D/85-D/70)}}} 

Can you simplify that compound fraction? If not post again asking how.
The D's will cancel out, so the answer is independent of the distance D.

Answer {{{108&2/11}}}km/h

Edwin</pre>