anonymous 4 years ago the radius of a circular puddle is growing at a rate of 20 cm/s. How fast is the area growing at the instant when it equals 25 cm^2? (use the area formula to determine the radius at that instant)

So, we have that $A = \pi r^2$ so the time derivative of the area is, via the chain rule, $\frac{dA}{dt} = 2\pi r \cdot \frac{dr}{dt}$