Question

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?

• A. 8
• B. 12
• C. 17
• D. 15

Answer 1

Answer:

B. 12

For a round trip over the same distance, the average speed is given by the harmonic mean of the two speeds:

$\text{Average speed} = \frac{2ab}{a+b}$
Speed from Kolkata to Ahmedabad (= 9) km/hr

Speed from Ahmedabad to Kolkata (= 18) km/hr

$\text{Average speed} = \frac{2 \times 9 \times 18}{9 + 18}$
$= \frac{324}{27}$
$= 12$

Average speed = 12 km/hr