Please have a look at diagram attached.
Let the height of the pole is H and its shadow is 72 ft.
The height of the fence is 3 ft and its shadow is 4 ft.
Both are similar triangles.
H/72 = 3/4
Cross multiplying
4 H = 72*3
4 H = 216
Dividing by 4
H = 54 feets : Answer
Hope that will help :)
