Answer: 4 ft/s
Explanation:
Given
height of man
speed of person 
height if street light
Let x be the distance between person and street light and y be the length of his shadow
From diagram
as the two triangle ADE and ABC are similar therefore we can say that




differentiate above Equation w.r.t time we get


The formula for working out speed is distance ÷ time.
55 km ÷ 2 hours = 27.5 km/h (average speed for first part of journey)
52km ÷ 5 hours = 10.4 km/h (average speed for second part of journey)
(27.5 + 10.4) ÷ 2 = 18.95 km/h (average speed throughout the journey)