A motorist travels to a place 100 kms away at an average speed of 50 km/hr and returns at 40 km/hr. The average speed of the journey is _________ km/hr?
The Usual Method
Since, speed is a dependent variable on distance and time; we cannot directly find the Arithmetic Mean (Average) of the values.
Average speed of the journey is calculated using the Harmonic Mean concept:
Average speed = where ‘1’ and ‘2’ are the two speeds of upward and downward journeys.
Hence, average speed in this case = = 44.44 km/hr
Estimated Time to arrive at the answer = 30 seconds.
The Arithmetic Mean (AM) of the values 50 and 40 is 45.
We already know that the Harmonic Mean (HM) of a set of numbers is always less than the AM of the same set. (i.e. HM < AM).
Hence, the answer should be < 45. The only option < 45 is 44.4 km/hr
Estimated Time to arrive at the answer = 5 seconds.