Multiple choice

A man covers a certain distance by bus, moving at a speed of 60 km./hr. and he returns to the starting point by train, running at the speed of 100 km./hr. Find the average speed of the whole journey? एक आदमी एक निश्चित दूरी बस से 60 किमी./घं.. की चाल से तय करता है और वह वापस प्रारम्भिक बिन्दु पर रेलगाड़ी से 100 किमी./घं. की चाल से आता है। पूरी यात्रा में उसकी औसत चाल ज्ञात कीजिए।

  1. 60 km./hr./किमी./घं.

  2. 65 km./hr./किमी./घं.

  3. 70 km./hr./किमी./घं.

  4. 75 km./hr./किमी./घं.
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D Correct answer
Explanation

When distances are equal, average speed = 2ab/(a+b) where a and b are the two speeds. Here: 2×60×100/(60+100) = 12000/160 = 75 km/hr. The average is always less than the arithmetic mean (80) because more time is spent at the slower speed.

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