Question

A student goes to his friends house with the speed of 30km/h. He finds that the house of his friend is closed. He comes back.immediately with speed of 40km/h.which of the following expressions is correct​

Answer 1

Given info : A student goes to his friend's house with the speed of 30km/h. He finds that the house of his friend is closed. He comes back immediately with speed of 40km/h.

To find : the average speed of the student is ...

solution : let the distance between a student and his friend's house is x km.

first case : he goes to his friend's house with the speed of 30 km/h.

so, time taken to reach the house, t₁ = x/30 hrs

2nd case : he comes back with speed of 40 km/h.

time taken to come back in his own location, t₂ = x/40 hrs

now average speed of student = total distance/total time taken

= (x + x)/(x/30 + x/40)

= 2x/{(40x + 30x)/30 × 40}

= 2 × 30 × 40/(70)

= 2400/70

= 34.286 km/h

Therefore the average speed of student is 34.286 km/h.