Average speed

A person travels from city 'A' to city 'B' with uniform speed of 60kms/hour and returns from city 'B' to city 'A' on the same route with an uniform speed of 40 kms/hour. What is the average speed for the entire journey traversed?


Answer:

If you unwittingly answer as 50 kms/hour (60+40)/2, is the right one, we can only feel sorry for you. The right answer is 48 kms/hour. Here is how the method works. Let the distance between cities A and B be 'x' kms. To go from A to B, speed is 60 kms/hour. Distance is x kms. Time taken for the journey A to B = x/60 hour. Similarly to return from B to A, speed is 40 kms/hour. Distance is' x' km. Therefore time taken for the return journey = x/40 hour. Now total distance traversed = x + x = '2x' kms. Now total time taken = [x/60 + x/40]hour= 5x/120= x/24 hour. Average speed for the entire journey = 2x/[x/24]=48 kms/hour.