Question

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?

• A. 78
• B. 81
• C. 84
• D. 85

Answer 1

Answer:

C. 84

Since the distance is the same both ways, we use the average speed formula for equal distances:

$\text{Average speed} = \frac{2ab}{a+b}$

where
(a = 78) km/hr and (b = 91) km/hr.

$\text{Average speed} = \frac{2 \times 78 \times 91}{78 + 91}$
$= \frac{14196}{169}$
$= 84 \text{ km/hr}$
Average speed of the entire journey = 84 km/hr