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Question

A car travelled the first one-third of the distance d at a speed of 20 kmh−1, the second one-third of the distance at a speed of 40 kmh−1 and the last one-third of the distance at a speed of 120 kmh−1. Determine the average speed of the car.

A
18 km/h
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B
27 km/h
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C
36 km/h
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D
45 km/h
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Solution

The correct option is C 36 km/h
We know that speed=distance travelledtime taken

Time taken by the car to travel first one-third of the distance =d320
Time taken by the car to travel second one-third of the distance =d340
Time taken by the car to travel last one-third of the distance =d3120

Average speed=Total distance travelledTotal time taken

Average speed=d3+d3+d3d320+d340+d3120
=dd60+d120+d360
=36 km/h

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