A pebble is dropped in still water, forming a circular ripple whose
radius is expanding at a constant rate of 10 cm/sec. Find a formula
giving the area enclosed by the ripple as a function of time. When the
radius is 20 cm, how fast is the area enclosed by the ripple
increasing?

I know the answer, I just need it written up in a step by step process
for stupid people.

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Well that's interesting. This question is actually from a quiz I took
recently and I need to understand the step by step process because I
got it wrong.