A man goes to Ahmedabad from Kolkata at a speed

A man goes to Ahmedabad from Kolkata at a speed of 9km/hr and returns to Kolkata at speed of 18km/hr, through same route. What is his average speed (in km/hr) of the entire journey?
एक व्यक्ति 9km/hr की चाल से कोलकाता से अहमदाबाद जाता है तथा उसी मार्ग से 18 km/hr की चाल से कोलकाता वापस आता है। पूरी यात्रा के दौरान उसकी औसत चाल (km/hrमें) ज्ञात कीजिए।

8 12 17 15

Detailed Solution & Logic

Correct Answer:

12

Speed going = 9 Speed returning = 18
Average speed = $ \dfrac{2ab}{a+b} $
$ = \dfrac{2 \times 9 \times 18}{9+18} $
$ = \dfrac{324}{27} = 12 $
Answer: 12 km/h

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