A car crosses a man walking at 6 km/h. The man can see the

A car crosses a man walking at 6 km/h. The man can see the things up to 450m only in one direction due to fog. He sees the car which was going in the same direction for 4.5 minutes. What is the speed of the car?

Time = Total distance / Relative speed
4.5/60 hr. = (450/1000) / x
⇒ x = 6 km/h

Relative speed = Speed of car - Speed of man
6 = x - 6
⇒ x = 12 km/h