Four-leg round trip — average over mixed speeds: A driver travels from City 1 to City 2 at 60 km/h and returns at 40 km/h. He then repeats the journey: going again at 120 km/h (twice the original onward speed) and returning at 20 km/h (half the original return speed). What is his overall average speed for the entire four-leg journey?

Introduction / Context:
Average speed over multiple legs depends on total distance and total time, not the average of speeds. With equal leg distances D each way, the computation simplifies.

Given Data / Assumptions:

• Leg speeds: 60, 40, 120, 20 km/h over equal distances D.
• Total distance = 4D.

Concept / Approach:
Compute total time as the sum of per-leg times, then divide total distance by total time.

Step-by-Step Solution:

Verification / Alternative check:
Pick D = 120 km; compute exact times; the ratio remains 40 km/h.

Why Other Options Are Wrong:
50–55 km/h arise from naïve averaging of speeds rather than distance/time aggregation.

Common Pitfalls:
Using arithmetic mean of speeds for round trips or mixed legs with unequal times.

Final Answer:
40 km/h