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CBSE Class 11-science Answered

a car travels first 3rd of a distance with a speed of 10km/hr , the 2nd 3rd of a distance at a speed of 60km/hr . determine the average speed over the entire journey?
Asked by meryprina | 10 May, 2013, 11:28: AM
answered-by-expert Expert Answer
Please note that in the question, you have only provided the speeds for the first and the second third of the distance and have not said anything about the last lap of the distance. Hence, we have considered only the first and the second third of the distance while calculating average speed. 
 
Average speed = total distance/total time taken
 
Let the total distance be x
hence, the car travels first thrid of the distance with the speed of 10km/h
Hence, distance travelled = x/3 km. 
Time taken = distance/speed = x/30 h
 
Now, the car travels second third of the distance with a speed of 60km/h
Hence, distance travelled = x/3 km
Time taken = x/3/60 = x/180 h
 
Hence, total distance travelled = x/3+x/3 = 2x/3
Time taken = x/30 + x/180 = 7x/180
 
Hence, average speed = 2x/3 / 7x/180 = 120/7 km/h
Answered by | 11 May, 2013, 10:26: AM

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