Charlie, who is 4 feet tall, walks away from a streetlight that is 15 feet high at a rate of 6 feet per second, as shown in the figure. Express the length s of Charlie's shadow as a function of time.

From similarity of triangles we have s/4 = (s+d) / 15 and so 4s = s + d and therefore s = d/3. Assuming he starts walking at t=0 from the street light (there is no figure to check this out) then d=6t and we then have s = (6/3) t