A girl goes to school at a speed of 6 km/hr. She comes back with a speed of 18 km/hr. Find her average speed for the whole journey.

Given:

Speed of the girl going to school = 6 km/h.

Speed of the girl coming back = 18 km/h.

Formula Used:

Average speed = Total distance travelled/Total Time

Calculation:

Let the distance to school be d km.

Time taken to go to school = d / 6

Time taken to come back = d / 18

Total Distance = d + d = 2d

Total Time = (d / 6) + (d / 18)

Total Time = d(1/6 + 1/18)

Total Time = d(3/18 + 1/18)

Total Time = d(4/18)

Total Time = \( \frac{2d}{9} \)

Average Speed = (2d) / \( \frac{2d}{9} \)

Average Speed = (2d × 9) / 2d

Average Speed = 9 km/h

The average speed for the whole journey is 9 km/h.