A man goes to Ahmedabad

A man goes to Ahmedabad from Kolkata at a speed of 78km/hr and returns to Kolkata at speed of 91km/hr, through same route. What is his average speed (in km/hr) of the entire journey?
एक व्यक्ति 78km/hr की चाल से कोलकाता से अहमदाबाद जाता है तथा उसी मार्ग से 91km/hr,की चाल से कोलकाता वापस आता है। पूरी यात्रा के दौरान उसकी औसत चाल (km/hr में) ज्ञात कीजिए।

78 81 84 85

Detailed Solution & Logic

Correct Answer:

84

Speed going = 78, Speed returning = 91
Average speed = same distance = $ \dfrac{2ab}{a+b} $
= $ \frac{2 \times 78 \times 91}{78 + 91} $
= $ \frac{14196}{169} $
= 84
Answer: 84 km/h

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