"A stone is dropped into a still pond. Concentric circular ripples spread out and the radius of the disturbed area increases at a rate of 16 cm/sec. At what rate does the area of the disturbed region increase when its radius is 4 cm?"

don't really know where to start.

I said A = πr^2 --> r = sqrt(A/π) --> d/dt(r) = sqrt(A/π) . d/dt

Now I'm lost...