A train travels from Delhi to Dehradun at the speed of 120 km/h and from Dehradun to Delhi at the speed of 130 km/h. What is the average speed during the whole journey?

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SSC GD Constable (2022) Official Paper (Held On : 13 Jan 2023 Shift 3)
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  1. 127.8 km/h
  2. 126.8 km/h
  3. 125.4 km/h 
  4. 124.8 km/h

Answer (Detailed Solution Below)

Option 4 : 124.8 km/h
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Detailed Solution

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Given values:

Speed from Delhi to Dehradun = 120 km/h

Speed from Dehradun to Delhi = 130 km/h

Concept:

Average speed = Total distance / Total time

Calculation:

⇒ If we assume the distance between Delhi and Dehradun to be d km, then total distance = 2d

⇒ Time taken from Delhi to Dehradun = d / 120, and from Dehradun to Delhi = d / 130

⇒ Therefore, average speed = 2d / ((d / 120) + (d / 130))

Hence, the average speed during the whole journey is approximately 124.8 km/h.

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