A girl goes to school at a speed of 21 km/hr. She comes back with a speed of 42 km/hr. Find her average speed for the whole journey.

Correct option is D

Given:

Speed while going to school = 21 km/hr

Speed while coming back = 42 km/hr

Formula Used:
The formula for average speed when the distances are the same is:

Average Speed =$$\frac{2 \times \text{Speed1} \times \text{Speed2}}{\text{Speed1} + \text{Speed2}}$$​

Solution:

Average Speed =$$\frac{2 \times 21 \times 42}{21 + 42} = \frac{1764}{63} = 28 \text{ km/hr}$$​

The average speed for the whole journey is 28 km/hr.