Correct Answer - Option 2 : 37.5 km/hr

Given:

Speed when travelling (1/3)rd of the distance = 25 km/hr

Speed for remaining distance = 50 km/hr

Formula used:

Time = distance/speed

Average speed = total distance/total time

Calculation:

Let total distance travel be D.

1/3 rd of distance = D/3

And remaining distance = [D - (D/3)]

⇒ 2D/3

Time is taken while travelling (1/3)rd of the distance = [(D/3)/25] hours

Time is taken while travelling remaining of the distance = [(2D/3)/50] hours

Then, Average speed = [(D/3) + (2D/3)]/[{(D/3)/25} + {(2D/3)/50}]

⇒ Average speed = [(D/3) + (2D/3)]/[(D/75) + (D/75)]

⇒ Average speed = [(D + 2D)/3]/[2D/75]

⇒ Average speed = (3D × 75)/(2D × 3)

⇒ Average speed = 75/2

⇒ Average speed = 37.5 km/hr

∴ The average speed of the entire journey is 37.5 km/hr.