A car travels up a hill at the constant speed of 40 km/h and returns down the hill at the speed of 60 km/h. Calculate the average speed for the round trip.

Let us assume that the distance travelled by the car in travelling up or down the hill is s. Let the time taken by the car to travel up the hill with speed v₁ is t₁ whereas the time taken by the car to travel down the hill with speed v₂ is t₂.
Given:
Speed with which car travels up the hill, v₁ = 40 km/h vl
Speed with which car travels down the hill, v₂ = 60 km / h
The average speed of the car for the round trip is given as:


The total distance travelled by the car in a round trip is twice the distance it travelled up or down the hill that is 2s.

….….….….….….….…...(1)
The elapsed time for the round trip is the sum of the time (t₁) taken to travel up the hill and time (t₂) to travel down the hill. Therefore the average speed is given as:
Also the time t₁ is given as:

The time t2 is given as:

Substitute the value of t₁ and t₂ in equation (1) to get:

Substitute the given values of v₁ and v₂ to get the average speed of round trip as:

Therefore the average speed of the car for the round trip is. 48 km/h.