A car overtakes a man walking at 6 km/h on a foggy road. The man can see objects up to 450 m ahead in his direction of travel. He continues to see the car (moving in the same direction) for 4.5 minutes after being overtaken. What is the car’s speed (in km/h)?

Introduction / Context:
After overtaking, the car pulls away. The man can see up to 450 m ahead; the car disappears when it is 450 m in front. The time to “disappear” equals distance / relative speed (car minus man).

Given Data / Assumptions:

• Visibility ahead = 450 m.
• Man speed = 6 km/h.
• Visible duration after pass = 4.5 min = 270 s.

Concept / Approach:
Let v be car speed (km/h). Relative speed = v − 6 (km/h) = (v − 6) * 1000 / 3600 m/s. Time = 450 / relative_speed.

Step-by-Step Solution:

Verification / Alternative check:
Relative 6 km/h covers 450 m in 270 s exactly.

Why Other Options Are Wrong:
9/12.5/15 km/h yield different relative speeds and times.

Common Pitfalls:
Using 450 m as two-way visibility or using man speed instead of relative speed.

Final Answer:
12 km/h