A car covers four successive stretches of 3 km each at speeds of 10 km/h, 20 km/h, 30 km/h, and 60 km/h, respectively. What is the average speed over the entire 12 km journey?

Introduction / Context:
When equal distances are traveled at different speeds, the average speed is found from total distance divided by total time—effectively a harmonic-mean type across the segments.

Given Data / Assumptions:

• Four segments, each 3 km (total 12 km).
• Speeds: 10, 20, 30, 60 km/h.

Concept / Approach:
Total time equals the sum of segment times d/vi. Average speed = total distance / total time.

Step-by-Step Solution:

Verification / Alternative check:
Harmonic-mean style: V = 4 / (1/10 + 1/20 + 1/30 + 1/60) = 20.

Why Other Options Are Wrong:
Arithmetic means (like 30 km/h) do not account for the longer time spent at slower speeds; 25 or 24 km/h are near-miss distractors.

Common Pitfalls:
Averaging the speeds directly instead of using time-weighted averaging via total time.

Final Answer:
20 km/h