I don't believe you should have divided by 2. You started with the formula of the area of one face of the plate, so your answer is for that one face of the plate, not both.
Question: A thin, circular plate in a boiler is expanding so that its radius is changing 0.01 cm/s. How fast is the area of one face changing when the radius is 5cm?
Here is my working, I'm I on the right track so far?
(this is for 2 plates - is this correct?)
Therefore, (is this correct?)