Rate of Change in surface area 
Question: $A$ sunflower grows so that its diameter increases at a
  constant rate of $0.1$ mm per hour until it reaches $200$ mm.  Find the rate
  of increase in the surface area of the flower when its diameter is
  $50$ mm.

How do I get the answer of $7.85 {(\mathrm{mm})^2}$ per hour?

Start with the formula relating its surface area (probably assumed to be the area of a circle) to its diameter. Take the derivative with respect to time, and don't forget the chain rule! Plug in the values you know, and solve for the one you don't.
