Application of differentation

Let distance of the man from the lamp post = s.
Let distance of the tip of the shadow from the lamp post = y.
By similar triangle,
540/y = 180/(y-s)
3/y = 1/(y-s)
3y - 3s = y
2y = 3s
dy/dt = [3/2] x (ds/dt).
Now ds/dt = 2, therefore,
dy/dt = 3/2 x 2 = 3m/s.

2008-07-29 15:44:17 補充：
I think it is the question that mis-typed 540cm to 540m. By common sense, how can a lamp post be 540 meters long!

Let y be the distance of the tip from the post,
Let x be the distance of the man from the post.

(y-x)/y = 1.8/540
300(y-x) = y
300(dy/dt - dx/dt) = dy/dt
299 dy/dt = 300(2)
dy/dt = 2.006689 m/s

2008-07-29 15:24:59 補充：
Answer 1 from eelwy is incorrect.

The height of post is 540m
The heigh t of man is 180cm = 1.8m

There ratio is 540/1.8 = 300 [not 540/180 = 3]

Is the height of the lamp post 540cm instead of 540m?