ripples on a pond - related rates

Relation:

`A=pi r^2`

Differentiate w.r.t. time, and then substitute known values:

`(dA)/(dt)=d/(dt)(pir^2)`

`=2pir(dr)/(dt)`

`=2pi(4)(0.5)`

`~~12.56 "m"^2"s"^-1`