An automobile travels from city A to city B and returns to city A by the same route. The speed of the vehicle during the...

Question

An automobile travels from city A to city B and returns to city A by the same route. The speed of the vehicle during the onward and return journeys were constant at 60 km/h and 90 km/h, respectively. What is the average speed in km/h for the entire journey?

A. 72 km/h  B. 73 km/h  C. 74 km/h  D. 75 km/h

Accepted Answer

Average speed = 72 km/h (option A). Average speed for a trip is total distance divided by total time, not the arithmetic mean of the two speeds. When the same distance is covered at two different speeds, the correct formula is the harmonic mean of the speeds. The onward and return legs each cover the same distance d, but the faster return trip takes less time.