SOLUTION: For the first hour of a four hour trip, the average speed of a train was 40 km/h. If the average speed for the whole trip was 70 km/h, what was the average speed for the last th

Algebra ->  Customizable Word Problem Solvers  -> Mixtures -> SOLUTION: For the first hour of a four hour trip, the average speed of a train was 40 km/h. If the average speed for the whole trip was 70 km/h, what was the average speed for the last th      Log On

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Question 690811: For the first hour of a four hour trip, the average speed of a train was 40 km/h. If the average
speed for the whole trip was 70 km/h, what was the average speed for the last three hours of
the trip?

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
average speed is distance/time
Let +d+ = distance traveled for the last 3 hrs of the trip
Let +t+ = the time in hrs for the whole trip
The distance for the 1st hour is
+40%2A1+=+40+ km
The distance for the whole trip is
+40+%2B+d+
The average speed for the whole trip was 70 km/h
The time for the whole trip was
+4+=+%28+40+%2B+d+%29+%2F+70+
+40+%2B+d+=+70%2A4+
+d+=+280+-+40+
+d+=+240+
+240+%2F+3+=+80+
The average speed for the last 3 hrs is 80 km/hr