Answer:
Tip of the shadow of the girl is moving with a rate of 7.14 feet per sec.
Step-by-step explanation:
Given : In the figure attached, Length of girl EC = 4 ft
            Length of street light AB = 25 ft
            Girl is moving away from the light with a speed = 6 ft per sec.
To Find : Rate ( ) of the tip (D) of the girl's shadow (BD) moving away from th
) of the tip (D) of the girl's shadow (BD) moving away from th
light. 
Solution : Let the distance of the girl from the street light is = x feet
Length of the shadow CD is = y feet
Therefore,  feet per sec. [Given]
 feet per sec. [Given]
In the figure attached, ΔAFE and ΔADE are similar.
By the property of similar triangles,

25x = 21(x + y)
25x = 21x + 21y
25x - 21x = 21y
4x = 21y 
y = 
Now we take the derivative on both the sides,

= 
= 
≈ 1.14 ft per sec.
Since w = x + y 
Therefore, 

= 7.14 ft per sec.
Therefore, tip of the shadow of the girl is moving with a rate of 7.14 feet per sec.