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in Distance, Time and Speed by (237k points)
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If a person divides a distance into 3 equal parts and travels the three parts with speeds of 78, 84 and 90 km/hr respectively, what is his average speed (in km/h) for the whole journey?
1. 87.3 km/h
2. 83.7 km/h
3. 84.3 km/h
4. 85.7 km/h

1 Answer

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Best answer
Correct Answer - Option 2 : 83.7 km/h

Let 3d be total distance of journey.

As we know, overall average speed of the car = (Total distance covered)/(Total time taken)

= 3d/(d/78 + d/84 + d/90) = 3/{(14 × 15 + 13 × 15 + 14 × 13)/( 6 × 13 × 14 × 15)}

= 3 × 16780/(210 + 195 + 182) = 3 × 16780/587 = 3 × 27.9 = 83.7 km/h

 

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