pwrinkle

I'm having problems doing this word problem and one similar to it.

A street light is mounted at the top of a 15 foot tall pole. A man 6 ft tall walks away from the pole with a speed of 5 ft/s along a straight path. How fast is the tip of his shadow moving when he is 40 ft from the pole?

I don't think I am doing the correct relation between the rates. I'm forming a relation using similar triangles and then deriving it and solving for the rate of change of the shadow.

If m=distance between man and wall, and s = distance between man and tip of shadow i form this relation.

15/(m+s)=6/s
use cross multiplicatoin
15s=6m+6s
9s=6m
derive to get rates of change
9s'=6m'
already know m'=5ft/s
9s'=6(5)
s'=3.333
3.333ft/s is wrong.

Could anyone please help me?

thanks