SOLUTION: 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

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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?
Found 2 solutions by stanbon, Edwin McCravy:
Answer by stanbon(75887)   (Show Source): You can put this solution on YOUR website!
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?
------------------------------
1st Half DATA:
distance = x km ; rate = 70 km/h ; time = d/r = x/70 hrs.
-------------------------------------------------------------
2nd Half DATA:
distance = x km ; rate = r km/h ; time = x/r hrs.
-------------------------------------------------------------
Total Trip DATA:
distance = 2x km; rate = 85 km/h ; time = (2x)/85 hrs
========================================================
Equation:
time + time = total time
x/70 + x/r = (2x)/85
1/70 + 1/r = 2/85
(r+70)/70r = 2/85
Cross-Multiply:
140r = 85r + 85*70
55r = 85*70
r = 108.18 kph (rate needed in the 2nd half of the trip)
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Cheers,
Stan H.
Answer by Edwin McCravy(20056)   (Show Source): You can put this solution on YOUR website!
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?

Let's suppose the whole trip is 2D km, since time = distance/speed, you
have to get there in  hours.  You have gone D km at 70 miles
an hour so again since time = distance/speed you have used up 
hours of the  hours, and you have only  hours
left to go the remaining D km.  Since speed = distance/time, the

speed =  

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 km/h

Edwin
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