You are 6 feet in height. If you are standing away from the base of the lamppost, you can see where your shadow stops. If you are 10 feet from the base of the lamppost and your shadow stops 5 feet from where you stand, what is the height of the lamppost?
As usual, draw a diagram.
if the lamp height is h, using similar triangles,
6/5 = h/(10+5)
h = 18